Popular Science Monthly/Volume 68/June 1906/Concerning Variations in Animals and Plants
|CONCERNING VARIATIONS IN ANIMALS AND PLANTS|
By PRESIDENT DAVID STARR JORDAN,
WITHIN any given species of animals or plants as occurring in nature, variations of many sorts may appear. No one individual is the exact image of another, either in structure or in function. In theory, at least, no one cell is exactly like another, no one chromosome the exact duplicate of a mother or a sister chromosome. Moreover, no one group or aggregation of individuals is exactly like another, if separated from it by time or space. In the classification of variations we may naturally divide them into individual variations and collective variations. Collective variations are produced by the extension of certain types of individual variations from generation to generation. These form the basis of new species, as gaps are produced within the series by isolation or by the death of intermediate forms.
Individual variations are again sharply divided into those which are inborn or blastogenetic, and those which are acquired or ontogenetic, produced by the direct influence of environment or by the reaction of the organism from external conditions.
The inborn variations arising from differences in the original germ cells, male, female or both, or from results of their combination or amphimixis, may be again subdivided as fluctuations, saltations, monstrosities and hybridizations. So far as we know it is alone from inborn variations, as perpetuated by heredity, as sifted by natural selection and as protected by bionomic isolation, that collective variations, nascent species and new species originate.
The small differences, numerous but slight, and never wanting, which distinguish one individual from another of the same species have been termed fluctuations. These occur in every individual, in every organ and practically in every direction. They are traceable, on the one hand, to differences in the germ cell, and in the process of mitosis or cleavage to which it is subject, and, on the other, to the fact of amphimixis, or double parentage, universal in the higher animals and plants. These fluctuations are hereditary, but their existence is easily obscured by the fact that the slight variations in the one parent rarely coincide with those of the other. In the progress of a species, individual fluctuations tend to neutralize one another.
It is, however, certain that fluctuations can be rapidly intensified and rendered stable by the process of selection, either natural or artificial. While it is probably true that few species originate by the process of selection alone, it is almost certain that it is possible for selection alone to produce groups equivalent to those we call species. There is apparently no limit to what man can do by the persistent preservation of favorable or desirable fluctuations. The same results must occur in nature when the same process takes place if it ever does take place. A slight advantage on the side of any special fluctuation will change the average variation in that direction and in time make the character permanent.
One of the first noteworthy studies of fluctuations as distinguished from climatic, environmental and geographic variations was that made by Dr. J. A. Allen in his paper on the 'Mammals and Winter Birds of East Florida,' published in 1871. In this paper Dr. Allen gives measurements of many specimens of various species of common birds, with a view to ascertaining the normal rate of variation. In this regard, the study of birds is much more instructive than that of most groups, because a bird of any species rapidly reaches a natural stature. It is affected in its growth by environment or food conditions much less than the members of most other groups. The well-fed bird reaches its normal stature, the ill-fed or injured bird dies, and the bird changes little with growth, its adult condition being once attained. Among adult male birds, characters due to accidents of condition are reduced to the minimum.
Dr. Allen finds that each part is subject to an ordinary variation of from 15 to 20 per cent, in its measurements, this in specimens of the same age and sex; at the same time each part varies independently of the others, even each feather of the wing and tail. Each toe may vary for itself, and the bill and the claws are subject to the same deviations from the normal. Color variations are equally well marked, and the variations within the species as represented in a single locality are often as great as those which actually distinguish species. In birds with streaks or spots, these markings vary in size, form and number, each individual having its own traits, which persist through the seasonal changes of plumage. It is part of the art of the faunal naturalist or systematist to determine which of these characters represent individual variations and which are associated with distinctness of species.
In general, the conditions of a species may be compared to a target filled with marks of shot. The bull's-eye represents the normal position of the shot, but variations in every direction occur. As one goes away from the bull's-eye the shot marks are fewer in number, but some of them may fall at a considerable distance from the center or average. Sometimes the majority may lie on one side of the center. This would indicate a continuous influence acting in one direction, as the wind affecting the direction of the bullets. Among animals a similar deviation from the usual may indicate a climatic effect, or the effect of natural selection under different conditions. That either of these influences may cause a mass variation within a group admits of no shadow of doubt. Nor is there any doubt that fluctuations within the species may and do constantly go as far as to equal the usual distinction between one species and another. The sole test of a species is in the relative permanence of its group of characters, not in the extent of its deviations from some other group. And in almost every case where the actual origin or cause of separation of one species from the next can be traced, it is found to have a geographic factor. Doubtless favorable fluctuations could be added together until those possessing them should be segregated from the mass about them, but we have no certain knowledge of any such cases. Doubtless individual fluctuations (saltations) could be so extreme as at once to segregate their possessor from all its neighbors. Doubtless new species might and perhaps do originate in that way, but out of the hundreds of thousands of species studied in nature, not one is certainly known to have had such an origin. But let any group of individuals be separated from their fellows, no matter how, in time the individuals remaining will come to an equilibrium with a different center of variation or a different arrangement of average characters from those possessed by the parent stock. The characters of any species represent the equilibrium or result attained by the forces of heredity and the operations of local natural selection.
We may again divide the traits of the fluctuations or minor variations of the individual into three classes, those useful, those indifferent and those harmful. A useful variation may cause the survival of the individual; a harmful variation may cause its destruction.
In considering the traits of a species, these classes of characters are reduced to two. Adaptive characters—those associated with the wellbeing of the individual—and non-adaptive, or indifferent characters—those which have no evident relation to utility. Characters positively harmful are always eliminated so far as the species is concerned.
In the actual work of the description of species, we recognize at once that adaptive characters are older than the non-adaptive characters. This is true, even though the adaptive character itself be a relatively recent one. The flying-fishes flew before the group was split up into thirty species. Yet the enlargement of the fins of these fishes is one of the latest products in fish evolution.
The species of Hawaiian birds of the family of Drepanidse are among the latest results of bird development. Yet these birds were divided into genera characterized by the form of the bill, each type of bill being adapted to a special purpose long before the present actual species were themselves differentiated.
All the orioles build hanging nests, and all are adapted alike in structure and instinct to their mode of life. All the sparrows and finches have bills fitted for cracking seeds. And these features of adaptation preceded the subdivision of orioles or sparrows into their present genera and species. In general, the traits by which we distinguish species are non-adaptive characters, while the features of adaptation are most distinctly traceable in structures common to many species, the characters of genera or of families in zoology.
But in this we find certain paradoxes. In studying the characters of members of a zoological family, we find that the most distinctly adaptive characters are relatively recent. It is a truism that physiological characters have a lower systematic value than characters not related to the character of life processes. The traits that fit animals for a special kind of food or for a special kind of topography are recognized as of low value in taxonomy. In other words, special adaptations are of relatively recent origin. General adaptations are older than special adaptations. Hence they have a higher value in classification. A flying-fish is fitted to swim in the water before it is adapted to leap in the air. But all general adaptations began some time as special ones. General adaptations have become so ingrained in the life of animals that they are, in a sense, invisible to the systematist. He passes them by as matters of course, directing his attention to special adaptations or to special peculiarities which seem to be devoid of utility. The ancient adaptations are not even considered as adaptations at all, when we say that adaptive or physiological characters have a low value in classification. And it is from this fact that the seeming paradox arises.
Again the adaptive character in the race or species may appear to be of very recent origin. The Southdown sheep of England had tawny faces before they acquired their present traits of wool or mutton. The explanation of such cases is this: an adaptation is never finished, a more rigid selection may at any time enforce a more rigid adaptation. Natural selection acts as a constant influence. It may be varied or intensified under new circumstances, as when it is directed by the will of man, when it becomes artificial selection. And under the influence of artificial selection a non-adaptive or indifferent character becomes adaptive or selective. Among those animals or plants which submit readily to domestication almost any natural species, distinguished by non-adaptive characters, could be reproduced with all its traits by a process of carefully controlled selective breeding.
It has long been known that individual fluctuations of an extreme degree sometimes occur, and that these may be to a degree persistent in heredity. Of such nature was the Ancon sheep, the iceberg blackberry and numerous other races or forms known in the domestication of animals or the cultivation of plants. The generally normal structure of such individuals distinguishes them from monstrosities, which are usually freaks of development rather than of heredity.
The name 'saltation,' or in recent years 'mutation,' has been applied to extreme fluctuation, the immediate cause of which is unknown. The experiments of Dr. Hugo de Vries on the saltations of the descendants of an American form of evening primrose (Œnothera lamarckiana) have recently drawn general attention again to the possibility that saltation has had a large part in the process of formation of species. As to this it may be said that the possible variation within each species is much greater than the range of the individuals which actually survive. The condition of domestication favors the development of extreme variation, because such individuals may be preserved from interbreeding with the mass, and they may survive even if their characters are unfavorable to competition in the struggle for existence. Among plants it is noticed that new soil and new conditions seem to favor large variation in the progeny, although the traits thus produced are rarely if ever hereditary. Cases more or less analogous to those noted by Dr. de Vries are not rare in horticulture. The cross-breeding of variant forms favors the appearance of new forms. Among actual species in a state of nature, there are very few which seem likely to have arisen by a sudden leap or mutation. The past and the future of de Vries's evening primroses are yet to be shown, and it is not at all unlikely that the original Œnothera lamarckiana found in a field near Amsterdam was a hybrid stock, a product of the florist, the behavior of its progeny being not unlike that which appears in the progeny of hybrids. The species called by de Vries Œnothera lamarckiana is not known in its wild state anywhere in North America, the parent region of the species of evening primrose or Œnothera. It is, moreover, known that the seeds of hybrids of an American species, probably Œnothera biennis, the common evening primrose, with other American species produced by Mr. Burbank, have been in past years sold in the cities of Germany. In any event, we have as yet no reason to assume that the various mutants of the evening primrose are in any sense comparable to the wild species of the same group now existing in America.
While saltation remains as one of the probable sources of specific difference, its actual relation to the process of species-forming in nature is yet to be proved.
Dr. de Vries's assertion that the process of natural selection is mainly a conflict between saltatory offshoots and not a competition between similar individuals ('intraspecific instead of interspecific competition') is hardly justified by the facts. The real conflict is that of the individuals maintaining life against the pressure of external conditions.
In the struggle for existence, each individual survives which can. The close presence of other similar individuals and that of unlike individuals are alike parts of the environment which each individual that leaves progeny has in some degree succeeded in conquering. This conquest takes place through adaptation to the actual conditions, concession to the actual environment.
It is highly probable that saltations in general are of the same nature as fluctuations, and that they occur in nature far more commonly than has been supposed. Unless in some way protected by isolation, the traits thus developed are likely to be swamped and lost by interbreeding with the mass. But it is conceivable that they are not always thus lost, as a very favorable variation may overwhelm the mass. But it is also clear that isolation of some sort would be usually, if not always, essential to any survival of a group possessing saltatory characters.
In crossing related species, new forms arise, having in part a blend or a mosaic of the characters of the two parent species, in part other traits or characters, the origin of which may not be clearly traced. Usually the second generation shows a great range of variation, often deviating farther in some respects from the average type than was the case with either of the original parents. The progeny of these variants may also vary widely. It has been sought to define the laws governing these variations. When only a few easily recognized characters are concerned, it has been possible to trace a certain regularity, conforming in general to the Mendelian law. But different species differ not in one way, but in a thousand ways. These traits are so interwoven and so variously related, some of them cumulative and some contradictory, that in most cases no law determining which characters shall be dominant in each case and which recessive can be made out. While the Mendelian law may be of universal application, it is in a few cases only that its facts can be clearly seen.
In heterogeneous hybridization or the crossing of distantly related forms, all sorts of results are reached, according to the nature of the forms in question. In few cases does the progeny become mature, and still more rarely does it prove fertile. If the plans of structure inherited from the two parents do not coalesce with reasonable completeness, nothing comes of the development of the progeny. Thus the strawberry may be readily crossed with the raspberry. The resultant plant grows and blossoms, but the divergent lines of heredity can not agree on the structure of the fruit, and the plant is wholly sterile.
In Mr. Burbank's cross of the European walnut (Juglans regia) with the California walnut (Juglans Californica), the first generation shows a certain blending of the traits of one species with those of the other. In the next generation appears every conceivable kind of variation in every feature of the plant and in every function of its organs.
Among artificial hybrids of different species a few are fertile and breed true to the parent stock. Among these are the Primus berry, a cross between the blackberry and raspberry, and the Logan berry, a chance cross between a cultivated raspberry and a California blackberry (Rubus vitifolia) growing outside a garden fence in Santa Cruz, California.
These two plants might be properly called distinct species, if we were not aware of the circumstances of their actual origin. But hybrids rarely form distinct species in nature. European writers have defined numerous hybrids among the fresh-water Cyprinidæ of the continent of Europe. Nothing of the sort occurs in the same group in America, and the evidence for hybridization needs at least re-examination.
Among American birds we have three notable cases: In the genus Helminthophaga (Helminthophila Ridgway), a group of American wood warblers, three nominal species have been described, each of which is usually regarded not as a true species, but as a hybrid of two of the common forms. One of these, called Helminthophaga cincinnatiensis Langdon, is 'obviously a hybrid' between Helminthophaga pinus and Oporornis formosa. Helminthophaga lawrencei Herrick, known from a few specimens, is perhaps a hybrid of Helminthophaga pinus and H. chrysoptera. Helminthophaga leucobronchialis Brewster, known from some scores of examples from various localities, is probably also a hybrid of the same two species. "It is probable," says Mr. Ridgway, "that both in the case of this form and H. lawrencei, that dichromatism, as well as hybridism, enters into the question of their origin. While H. pinus apparently exhibits rarely a white and gray phase (instead of olive and green) and H. chrysoptera as rarely a yellow and olive green instead of white and gray phase, the two species interbreed to such an extent not only with one another, but each with H. leucobronchialis and H. lawrencei (the hybrids being fertile inter se), that the problem is a very complicated one and therefore most difficult to work out satisfactorily."
Mr. W. E. D. Scott has lately maintained that these forms or species of warblers are not hybrids, but the results of saltations or mutations in the sense in which the word is defined by Dr. de Vries. But this view of the case finds little favor among ornithologists who continue to regard these forms as true hybrids.
In a single genus of marine fishes, the pargo or snapper (Lutianus), the types of certain nominal species have been suspected to be hybrids, because they show a perfect blend of the traits of well-known and related species found in the same waters. Of Lutianus lutianoides, from Cuba, one specimen is known. It is thought to be a hybrid of Lutianus apodus and Ocyurus chrysurus. Of Lutianus brachypterus, from New Providence, there is one specimen, thought to be a hybrid of Lutianus griseus and Lutianus synagris. Two specimens of Lutianus ambiguus have been found in Cuba. It is thought to be a hybrid of Lutianus synagris with Ocyurus chrysurus. Nothing is known as to the fertility of any of these forms. The record of such hybrids shows how rare is the occurrence of this phenomenon among fishes.
Along the boundary line of the western distribution of the golden winged flicker (Colaptes auratus) of the eastern United States, the species overlaps the range of the red-shafted flicker (Colaptes cafer), a species which replaces the first in California and Mexico. Among other differences, the former has the shafts of the wings and tail yellow. The other has these shafts red. But where the species meet individuals occur variously mixed. Some even have the tail red on one side and yellow on the other. These individuals have been recorded as hybrids (Colaptes auratus hybridus), and this origin is in fact probable.
In the Sonoran region, along the meeting line of Colaptes cafer (var. collaris), similar crossing with a Mexican golden woodpecker (Colaptes chrysoides) is said to take place.
Hybrids of wild plants sometimes occur, but the cases are rare, and in general it may be doubted whether hybridization has had any appreciable part in the origin of species.
The individuals possessing any sort of variations, large or small, fluctuation or mutation, are subject to the struggle for existence. This is the test of fitness. To live and to leave progeny means in itself a high degree of adaptation. Every form of struggle, as indicated above, is a struggle with the conditions of life, the competitive features, however, being inevitable accessory conditions. All struggle is in fact intraspecific, that of individuals within the species. When members of related species enter into competition in the same region, the struggle becomes in a degree interspecific, one between the species themselves through their actual representatives. But as there is practically no cooperation among members of the same species, except in the family or band relation of the higher animals, the struggle must be in fact always individual. The struggle against the environment in general is in its essence non-competitive, but in the crowd of animals and plants competition becomes part of the environment. Some seeds, we are told, fell on stony ground, but the plants died because they had no depth of earth. Some fell among thorns, and the thorns grew up and choked them. Still others doubtless perished because they were planted too close together. These three causes of the destruction of the great body of animals and plants, the competition with like forms, that with unlike forms, and the pressure of the environment, are present everywhere with all life in varying degrees, and by it all life is forced into lines of adaptation.
With vital characters, selection preserves those which have no utility by the simple action of heredity. All Southdown sheep have tawny faces, although this trait bears no relation to the short firm wool or to the fat and tender flesh, for which traits the Southdown sheep are bred and valued.
In similar fashion, many indifferent characters are traceable in the various breeds and strains of domesticated animals and cultivated plants. We may presume that similar characters in wild animals and plants have been similarly carried along by inheritance from ancestors possessing them. This phenomenon I have elsewhere called 'the survival of the existing.' The actual traits are reproduced by heredity without regard to any question of fitness.
The phenomena of dichromatism belong to the category of individual variation. In the vast majority of animals, we have the dimorphism of sex. In the beginning the embryo is sexless. From the beginning, by forces imperfectly understood, its development must be directed in one way or another. It must assume the structures and functions of one or the other sex. With certain insects, a polymorphic condition exists. With bees, the caste of workers or atrophic females exists, together with the type of drone and queen. With some ants, still other types of individuals occur, but the dimorphism of sex, or the polymorphism of the division of labor, rests on influences perhaps not altogether inherent in the structure of the parent cell.
In dichromatism, the individual from the first shows one or another of two color patterns peculiar to the species. The screech owls are some gray, some rusty red, within the same species, even in the same nest. In the same fashion, gray squirrels and black squirrels belong to the same species. In the same species of globe fish, Tetraodon nigropunctatus, some individuals are gray, and some citron yellow. In another species, Tetraodon setosus, some are gray, some yellow and some deep blue.
In a West Indian species of bass-like fish, strangely misnamed. Hypoplectrus unicolor, an extraordinary polychromatism occurs. All the known individuals belonging to one or another of the following color forms which have been called species:
|a.||Soft dorsal checkered or spotted with pale blue or crossed by blue lines (these occasionally obsolete).|
|b.||Body dusky, the head and belly orange, the top of the head olivaceous; a black spot on each side of caudal peduncle close behind dorsal; black band or spot in front of eye not bordered by blue; cheeks, opercle and breast with vertical lines of metallic blue; dorsal yellowish; pectoral and caudal orange; a black spot in the axil; upper margin of pectoral blue; anal orange with blue border; ventral greenish, its base orange.Unicolor (= Maculiferus).|
|bb.||Body all violet with 5 or 6 more or less distinct black cross bands, the middle one broadest, covering the space from the fourth to the tenth dorsal spine and meeting its fellow under the belly; the band at the nape broad and saddle-like, bounded by 2 pale cross streaks on nape, opercle and cheeks; snout pale, a pale shade across it; ventrals pale or dark; other fins, except spinous dorsal, mostly pale.|
|c.||Cheek with a blue band before eye and some blue spots before it.Puella.|
|cc.||Cheek without blue band; no blue spots on snout; colors duller.Vitulinus.|
|bbb.||Body and head yellow anteriorly; body abruptly black posteriorly, the black extending forward to a wavy line reaching from first dorsal spine to vent; a broad dark-blue band in front of eye, bordered by sky blue; fins chiefly orange; ventral and anal bordered by sky blue.Pinnavarius.|
|aa.||Soft dorsal plain, without distinct blue lines or spots.|
|d.||Preorbital region with 1 or more dark-blue stripes, bordered by bright sky blue (not fading in spirits).|
|e.||Body yellow anteriorly, black posteriorly, the black extending forward to a line joining the nape and last anal ray; fins orange; a single blue-black stripe or spot in front of eye, ocellated with sky blue; caudal peduncle very dark above.Guttavarius.|
|ee.||Body all orange yellow, fins orange; snout and lower jaw blue; 2 blue stripes, each bordered with sky blue, before the eye.Gummigutta.|
|eee.||Body saffron yellow, orange posteriorly; snout with blue streaks and some blue dots.Crocotus.|
|dd.||Preorbital region without blue stripes.|
|f.||Preorbital region with violet spots; a round black spot on side of caudal peduncle; dorsal light greenish; body light olive green above, reddish below; pectorals pale yellow, the first ray blue; ventrals, anal and caudal light orange.Aberrans.|
|ff.||Preorbital region without distinct violet spots. |
|g.||General color blackish, brown or yellowish—not indigo blue.|
|h.||Color brownish, the middle of the front of body yellowish; fins all yellow except the ventral, which are black.Accensus.|
|hh.||Color yellowish pink; caudal and pectorals pale; ventrals and anal bright light blue.flinis.|
|gg.||Color of body black with violet shades.|
|i.||Pectoral and caudal fins abruptly bright yellow.Chlorurus.|
|ii.||Pectoral and caudal fins violet black like the rest of the body. Nigricans.|
|j.||General color deep indigo blue everywhere on body and fins; body with 4 to 6 broad cross bars of darker blue.|
|k.||Cheeks plain, without distinct stripes. Indigo.|
|kk.||Cheeks with a dark-blue suborbital band between 2 bands of clear blue.Bovinus.|
What the significance of this extraordinary condition may be, is entirely unknown, nor do we know the determinant causes of dichromatism. It is sufficient for the purposes of this paper to refer to the fact.
A vast range of variations are ontogenetic, or dependent on influences affecting directly the life of the individual. These are not hereditary in the judgment of the present writer, and therefore they are not direct factors in the formation of species. Many investigators take a different view, believing in the direct inheritance of acquired characters, or of the effects of environmental or functional conditions, the familiar tenets of 'Neo-Lamarckism' or progressive heredity.
These ontogenetic variations are, strictly speaking, individual, appearing as collective only when many individuals have been subjected to the same conditions. They may be divided into environmental variations and functional variations, two categories which can not always be clearly separated, as variations due to food conditions partake of the nature of both.
In the epoch-making paper quoted above, and in other publications, Dr. Allen shows that climatic influences affect the averages in measurements and in color among birds. For example, in several species of birds, the total length is greater in specimens from the north, while the bills and toes are actually longer in southern specimens. That this condition is due to the influence of climate on development is shown by the fact that numerous species are affected in the same way. It is noticed also that specimens from the northeast and the northwest of the United States are darker in color than those from the interior, and again that red shades are more common in the arid southwest. Similar effects have been recently shown by a study of species of wasps. They may be produced at will by subjecting the larvæ and pupæ of insects to artificial heat and cold. The butterflies of the glacial regions and those developed in an ice-chest have a pale coloration, and a warm environment deepens the pigment.
It has not been shown that any of these effects are hereditary, or that they constitute a factor in the formation of species, although climatic effects may enter into the process of natural selection. I have before me a series of woodpeckers selected by a student (Mr. Hubert Jenkins), which illustrates at once climatic and other subspecific variation. The collection represents the species known as hairy woodpecker (Dryobates villosus). Taking the typical form villosus, from the eastern United States, we note that specimens from further south (auduboni) are smaller in every way, but otherwise similar. To the northward, in Canada and on the Arctic Sea, the birds are much larger, (leucomelas) ten to eleven inches in length instead of eight to nine, while the feet are scarcely if at all enlarged. In all these the space before the eye is black, and the belly is darker in specimens from the region having the most rainfall. In the Bahamas is a form still smaller, seven to eight inches long (maynardi), with the space behind the bill white. Further westward, in all woodpeckers of this species, the white spots on the wing coverts, characteristic of the eastern forms, nearly or quite disappear, leaving the feathers plain black. Here again the northernmost forms are largest, harrisi of the Californian-Alaskan region being nine to ten inches long, those from California being nearly white below, those from the Vancouver region smoky gray, darkest when the rainfall is greatest. The Mexican form (jardini) is seven to eight inches long, but in the moist regions of Central America it too becomes deep smoky brown. Of these traits, those relating to the size of the bird and the smoky coloration of its lower parts may probably be regarded as climatic. Whether a bird born of northern parents would reach its full stature in the south, or whether it would grow up with white belly plumage in a rainy district, are both open to question. The experiment can hardly be tested with woodpeckers, but some other group may offer conditions more favorable to artificial breeding.
On the other hand, the loss in the western birds of the white wing spots characteristic of villosus and its subspecies, leucomelas, auduboni, and maynardi, can have apparently no climatic cause, but is one of the results of the primitive separation of the forms on the two sides of the Rocky Mountains, or more properly of the treeless plains where woodpeckers of this type are not found.
It may be noticed that in the related but much smaller American species known as the downy woodpecker (Dryobates pubescens), the eastern forms (pubescens) have also the wing coverts profusely spotted with white, while in the western form (gairdneri) the wing coverts, as in the western forms of Dryobates villosus, are nearly or quite plain black. The ornithologists have not yet agreed on a subdivision of this species into larger northern or smaller southern subspecies, nor have the darker specimens from regions of greater rainfall received distinct names. Similar variations have been noted in the Hudsonian chickadee (Parus hudsonius) and in other birds of the northern forests. To what extent the variations seemingly due to direct influence of climate are hereditarv has not been ascertained.
While the embryo may be deprived of its normal growth momentum, the effect of unfavorable conditions is spent with a few generations of normal life, and no direct change in the heredity of the race is known to arise from the direct effects of environment.
Food Variations of Silkworms
Elaborate experiments in the effects of the underfeeding of silkworms have been made by Professor Vernon L. Kellogg and Mrs. Ruby Green Smith. The following synopsis of the results is given by Mrs. Smith.
|1.||Larval moltings, pupation and emergence of adult delayed and metamorphosis thereby prolonged.|
|2.||Fertility reduced (as indicated by number of eggs laid, number hatching and number of individuals reaching maturity).|
|3.||Mortality in all stages greater than among normally-fed individuals.|
II. Variations from the normal which are subject to quantitative measurements.
|1.||Reduction in size and weight of all parts in all stages (exemplified statistically and quantitatively by larval widths and lengths, by moth wing expanse, and by larval, pupal and adult weights).|
|2.||Reduction in quantity of silk produced, the cocoons being below normal in dimensions, thickness and weight.|
|3.||Degeneration of wing veins slightly more marked than in normally-fed individuals (economy of material being practised to advantage in these comparatively useless structures).|
The results of underfeeding in heredity have been studied in three generations derived from the original underfed great-grandparental stock. The characters which the underfeeding was known to affect were studied in a comparative way in 1904 among numerous lots.
Heredity of underfeeding among silkworms whose make-up may be summarized as follows:
|1.||Lots accustomed always to years of plenty.|
|2.||Lots in which some one of the four generations experienced famine|
|3.||Lots in which famine was experienced in two alternate or two successive years.|
|4.||Lots in which famine was experienced in three successive or in one alternate and two successive years.|
Throughout these lots the conclusions were consistent, and were in brief as follows:
|1.||The lingering effects of a single generation of underfeeding may be definitely traced to the third generation, although the progeny of the underfed generation be given the optimum amount of food. |
|2.||The power of recovery through generous feeding exhibited by the progeny of individuals subjected to famine is so extensive that three generations 01 plenty succeeding one generation of famine are sufficient to bring about the complete recovery of the race from the dwarfing consequent upon a generation of famine. Thus individuals of the fourth generation—the great-grandchildren of the starvelings—are the compeers in every respect of individuals of normal ancestral feeding. It is highly probable, but not yet proved, that recovery is possible even after three generations of famine.|
|3.||The effects of famine grow less evident the further removed (in heredity) the individuals are from its occurrence in their ancestral history. Thus, in two lots having but one underfed generation in 4, a lot having the underfed generation 2 or 3 years previous would rank higher in all respects than one whose immediate parents were its only underfed ancestors.|
|4.||A fourth generation on insufficient feeding has not yet been reared successfully in two years' trial. It is highly probable that the race cannot survive more than three generations of poor nourishment.|
It will be noticed that the differences between the normally fed individuals and those subjected to famine are not species-making differences, as specific characters go with the Lepidoptera, there being no differences in color or patterns, or shape or venation of wings, or larval or adult ornamentation. If a species or race of silkworms were named on the basis of characters induced by famine, it would be a 'size' and 'season' species—a Lilliputian race of silkworms having a lengthy metamorphosis.
While at first glance these experiments might seem to offer an instance of the inheritance of acquired characters, it is, however, apparent that the underfeeding affects the nourishment and full development, not only of visible parts of the body, but also of the germ cells and all internal parts of the body. The germ cells need not be said, therefore, to have been influenced by the acquired somatic characters and to have transmitted them as such, but rather they may be said through their own lessened vitality to have produced progeny with characteristics so parallel to those of the parent soma as to make it appear an authentic case of the inheritance of acquired characters. We have, therefore, a case of transmission of imperfect nutrition, not one of true heredity, a distinction made clear by Weismann. Moreover, if acquired characters are really hereditary, their inheritance should last for more than three generations.
One interesting result of this experiment is that (in so far as silkworm testimony goes) temporary trying conditions do not handicap the race in the long run. It is even conceivable that the ultimate result of famine might be a strengthening of the race (physically speaking), the famine playing the part of a selective agent, preserving only the strong and adaptable.
Of like nature, but often far greater in degree, are the changes in the individual dependent on differences in food, in nurture or in surroundings generally. In the life of a plant the environmental variations may be so great as apparently to overshadow all the innate characters or peculiarities. With the higher animals the direct effect of environment is proportionately less, and it reaches its minimum among birds and the more specialized insects. Yet no individual of any species is without some traits of variation due entirely to environmental influences. In fact heredity does not repeat the traits of the parent, but merely the tendency to develop similar traits under similar conditions. Change utterly the conditions of growth, and the same heredity will show itself in very different results. Strictly speaking, the characters of a species are not those which appear, but the ability to develop these characters under the conditions surrounding the ancestry of the individual.
But there is no evidence that the direct influence of environment is a factor in the separation of species, except as its results may be acted upon by natural selection. We have no reason to suppose that the environment of one generation determines the heredity of the next. It is true in a broad way that the ill-nourished offspring has weaker or less numerous offspring, but weak or strong, their hereditary traits are those of their actual parent stock.
The features of the 'ontogenetic species' or subspecies, have long been known under the name of 'convergence of characters,' 'parallelism' and 'analogous variation.' An 'ontogenetic species' is a group in which the likeness among the members is due, not to genetic connection, but to the exposure of the individuals to like conditions of development. Hence it should have no recognition in taxonomy, which deals with phylogenetic species and subspecies only. But no species is truly defined when only the usual characters, those developed under usual conditions, are considered. To know the species fully, we should know what qualities individuals may develop under all the varied relations of the environment in which they may be placed.
Ontogenetic species, however, tend to become phylogenetic, in isolation from the rest of their kind, by interbreeding among themselves, and under new conditions of selection. The real characters of the race thus formed may be wholly obscured by the more evident characters due to food conditions or to reaction from the environment. To test the characters, phylogenetic and ontogenetic, and to purge the system of species and subspecies founded on the latter, will be part of the work of the student of species for a long time to come.
Taking concrete illustrations—the Loch Leven trout, Salmo levenensis, recently discussed, is distinguished in its native waters by certain obvious characters. These disappear when the eggs are planted in brooks in England or in California, and the species develops as the common English brook trout. But it is conceivable that the obvious or ontogenetic traits of the Loch Leven trout are not the real or phylogenetic distinctions, and that the latter, more subtle, engendered through individual variation, inheritance, selection and isolation, really exist, although they have escaped the attention of ichthyologists.
After the species was planted in Yosemite Park in 1896, it remained for nine years unnoticed. In 1905, individuals sent to me were, so far as I could see, exactly like English brook trout. But it is conceivable that differences in food and water have caused slight ontogenetic distinctions. It is certain that in isolation from all parent stocks they will, in time, develop larger differences, which, after many thousand generations, will be specific or subspecific. At present, these trout are quite unlike the native rainbow trout (Salmo irideus gilberti) of the Yosemite. The ontogenetic characters will perhaps approach those of the latter, but the phylogenetic movement may be in quite another direction.
Another ontogenetic species is the little char or trout, Salvelinus tudes Cope, from Unalaska. In Captain's Harbor, Unalaska, the Dolly Varden trout, Salvelinus malma, swarms in myriads, in fresh and salt water alike, reaching in the sea a weight of six to twelve pounds. A little open brook, which drops into the harbor by an impassable waterfall, contains also an abundance of Dolly Varden trout, mature at six inches and weighing but a few ounces. This is Salvelinus tudes. In the harbor the trout are gray with lighter gray spots, and fins scarcely rosy. In the brook, the trout are steel blue, with crimson spots and orange fins, striped with white and black. In all visible phylogenetic characters, the two forms of trout are one species. We have reason to believe that fry from the bay would grow up as dwarfs in the brook, and that the fry from the brook would be gray giants if developed in the sea.
But it is also supposable that in the complete isolation of the brook fishes, with free interbreeding, there would be some sort of phylogenetic bond. There may be a genuine subspecies, tudes, characterized not by small size, slender form and bright colors, but by other traits, which no one has found because no one has looked deeply enough.
In no group of vertebrates are the life characters more plastic than among the trout. The birds have traits far more definitely fixed. Yet differences in external conditions must produce certain results. I should not venture to suggest that the dusky woodpeckers or chickadees of the rainy forests of the northeast and northwest are purely ontogenetic species or that they should be erased from the systematic lists. But it will be a great advance in ornithology when we know what they really are and when we understand the real nature of the small-bodied, large-billed, southern races of other species of birds. It would be worth while to know if these are really ontogenetic purely, or if they are phylogenetic through 'progressive heredity,' the inheritance of acquired characters, such as the direct effects of climate or as the reaction from climatic influences. Or again, may there be a real phylogenetic bond through geographical segregation, its evidences obscured by the more conspicuous traits induced by like experiences? Or are there other influences still more subtle involved in the formation of isohumic or isothermic subspecies?
Functional variations are variations produced in the individual by the use or disuse of organs. They are most marked in the most active organisms, notably in man, in other mammals and in birds. They form a large factor in the development of the individual. The education and training of the individual man produces functional variations, as distinguished from innate peculiarities. But the groups characterized by functional variations are of the nature of 'ontogenetic species.' There is no evidence that the current of heredity is affected by changes of this kind, or that they have any direct effect in the formation of true species. At the most, use or disuse of organs seems to affect species only in an indirect way, as by the preservation of those most disposed to functional activity, and which by such activity have been able to meet better the demands of the environment.
The preservation of individual variations with their extension to posterity gives rise to racial changes, and these to the larger variations which mark change in species.
Collective variations are chiefly of geographical or geological origin: Changes with space produce geographical species and subspecies. To this category belongs the vast majority of species and subspecies recognized in systematic zoology and botany. It is illustrated by the species of wood warbler (Dendroica) mentioned in a previous paper.
Changes with time produce geological mutations. It is a fact unquestionable that a species will change on its own grounds little by little with the lapse of time and the slow alteration of conditions of selection. Nations change, languages change, customs change, nothing is secure against the tooth of time. This is in general true, because with time alteration of environment takes place, events happen, there is an alteration of the stress of life and with this alteration all life may be modified.
That time-mutations in all forms of life do take place is beyond question, and some have regarded these slow changes as the chief agency in the formation of species. But the current of life does not flow in straight lines nor in an even current. Species are torn apart by obstacles, as streams are divided by rocks, and the rapidity of their formation is proportioned to the size of the obstacle and the alternations it produces in the flow of life.
We have some basis for the estimate of the duration of a species. When the great glacial Lake Bonneville occupied the basin of the Great Salt Lake, the same species of fishes and insects were found in all its tributaries. Now that these streams flow separately into a lifeless lake, the same species of fishes occur in them for the most part without alteration. One species of sucker (Catostomus ardens) and one chub (Leuciscus lineatus) are found unaltered throughout this region and in the Upper Snake River (above Shoshone Falls), into which Lake Bonneville was once drained. Other species are left locally isolated, but in one species only (Agosia adobe) a small minnow of the clay bottoms can be shown to have undergone any alteration. But with the tiger beetle (Cicindelœ) a large number of species have been produced by isolation.
From the Bay of Panama 374 species of fishes are recorded in the recent monograph of Gilbert and Starks. Of these species, 204 are recorded also from the Gulf of California, while perhaps 50 others are represented in the more northern bay by closely related forms.
Comparing the fish faunas separated by the isthmus, we find the closest relation possible so far as families and genera are concerned. In this respect the resemblance is far closer than that between Panama and Chile, or Panama and Tahiti, or Panama and southern California. On the Atlantic side, similar conditions maintain, although the number of genera and species is far greater (about 1,200 species) in the West Indies than at Panama. This fact accords with the much larger extent of the West Indies, its varied groups of islands isolated by deep channels, and its near connection to the faunas of Brazil and the United States.
But it is also noteworthy that while the families of fishes are almost identical on the two shores of the isthmus of Panama, and the great majority of the genera also, yet the species are almost wholly different.
Taking the enumeration of Gilbert and Starks, we find that out of 374 species, 43 are found apparently unchanged on both sides of the isthmus; 265 are represented on the Atlantic side by closely related species—in most cases the nearest known relative of the Pacific species—while 64 have no near analogue in the Atlantic. Of the last group, some find their nearest relative to the northward or southward along the coast, and still others in the islands of Polynesia.
The almost unanimous opinion of recent students of the isthmus faunas finds the latest expression in the following words of Gilbert and Starks ('The Fishes of Panama Bay,' p. 205):
It is obvious, however, that the striking resemblances between the two faunas are shown as well by slightly divergent as well as by identical species, and the evidence in favor of interoeeanic connection is not weakened by an increase in the one list at the expense of the other. All evidence concurs in fixing the date of that connection at some time prior to the Pleistocene, probably in the early Miocene. When geological data shall be adequate definitely to determine that date, it will give us the best known measure of the rate of evolution in fishes.Of the 82 families of fishes represented at Panama, all but three (Cerdalidæ, Cirrhitidæ and Nematistiidæ) occur also on the Atlantic side of Central America, while of the 218 genera of the Panama list, no fewer than 170 are common to both oceans. The well-developed families, Centropomidæ and Dactyloscopidæ, are peculiar to the two tropical faunas now separated by the Isthmus of Panama. It might be added that the families of Nematestudæ (one species) and Cerdalidæ (three species) are confined to the Panama region, while the Cirrhitidæ (one species) belongs to a group characteristic of the islands of Polynesia.
From this discussion, it is probable that even in isolation some species change very slowly, that with similar conditions the changes within isolated groups of a species may be parallel, and that the specific changes in different groups may progress with very different degrees of velocity. Natural selection apparently furnishes the motive power of change, but the initiative comes from variation and heredity, and its direction and final results depend on a multitude of conditions and circumstances of environment which are largely geographical, topographical or climatic in their nature.
Topographical segregation may bring about the separation of subspecies or species in precisely the same manner as other methods of geographical isolation. An example is that of the deep-water trout of Lake Tahoe, Salmo henshawi tahoënsis. The ordinary Tahoe trout, Salmo henshawi, lives in the shallow parts of the lake, spawning in the streams. This form, larger in size, more robust in form and less spotted in color, lives in the depths of the lake, spawning near the shore. The difference between the two is not great, but is perhaps sufficient to justify the subspecific name (tahoënsis). The two are considered as different species by anglers.
A more strongly marked case, probably of earlier origin, is seen in several West Indian species of grouper or sea bass, belonging to the genus Mycteroperca. In these species, the shore forms have an olivegreen color, while others, essentially similar, in deep water are crimson or scarlet. Thus Mycteroperca venenosa, the yellow-fin grouper, has a scarlet cognate form, Mycteroperca venenosa apua, Mycteroperca tigris, likewise green in shallow water, has its deep water representative in M. tigris camelopardalis. The same condition holds with Mycteroperca olfax of the Galapagos and its cognate M. olfax ruberrima. In another species of this type, the Guatívere, Cephalopholis fulvus of the West Indies, the shore form is dark olive, (C. fulvus punctatus), that found in deeper water is crimson (C. fulvus ruber), while in still deeper water is a golden-yellow form, the original Cephalopholis fulvus. Similar relation of olive and red forms, or of black and yellow types, has been noticed in other groups of fishes.
In similar fashion, it is claimed that within the species lines of segregation may be set up on a physiological basis those of a certain type not breeding freely with those of other types. This theory is largely hypothetical. It is conceivably true under certain circumstances, but there is little evidence that any particular species has originated in that way.
It is beyond question that differences greater than those ordinarily separating distinct species may be produced by continuous conscious or unconscious selection on the part of man. To what extent these breeds would retain their characters under the leveling processes of nature, it is impossible to say in any particular case. On the other hand, in the nature of things, all of them are of very much more recent origin than competing species in nature. While it may be that no wild species has originated from human selection, it is true, on the other hand, that the races thus produced are the same in essence as the subspecies and species produced in nature. The same forces are at work, the basis of selection only being altered by man. The river flows according to the same laws over a natural ledge of rock or over an artificial dam.
Hybridization, as above shown, may produce species as well marked and as fertile as any natural species. But no wild species is known to have arisen from this cause, unless we regard the warbler, Helminthophaga leucobronchialis, as an established species.
A saltation or mutation, beginning with an individual, may extend its characteristics to a numerous progeny, thus forming the beginning of a species. But while this influence may in theory, or even in fact, have a large importance, it is not likely that many species originate in this way. There is no clear evidence that any wild species known to us has arisen from a sudden large individual variation or mutation.
A large number of unexplained, but apparently related, phenomena have been recorded under the name of determinate variation, or have been grouped together as examples of a process to which Eimer has applied the name of Orthogenesis.
Setting theory aside, these cases are essentially of this character. In geological times a certain number of genera appear, each one in a certain direction farther along than its predecessor. Very often a certain organ will be progressively more and more highly developed until a certain point, when it is progressively degraded or simplified in its structure. Examples of this are found among the ammonites, or cephalopods with coiled shells, the chambers in the cell elaborately sculptured. Many illustrations are found in the group of fishes. Coats of mail may be built up step by step, genus after genus, and then gradually modified or abandoned. In the Chætodon-Zanclus-Acanthurus-Balistes-Tetraodon-Diodon-Mola series of fishes, we have a high development and specialization of the spinous dorsal followed by its entire loss step by step, with that of the ventral fin also. The scales, at first normal, are specialized to lancets, bony plates, spinules, and then gradually reduced to mere prickles and finally lost.
In the Cirrhitus-Sebastes-Scorpcena-Cottus-Psychrolutes-Cyclopierus-Liparis-Paraliparis series, we have a higher and higher specialization of fins and scales, with the final loss of the latter and a reduction of the fins to their lowest terms. Similar series connect the typical sharks with the rays. Other series among fishes begin with specialized forms, but end in the degeneration seen in multiplied and unspecialized vertebræ and fin rays. The well-known horse series and the series of monkeys and apes—each genus in certain lines being progressively more anthropoid—may be considered in this connection. In fishes, many of these series may be clearly traced among forms still existing, the most primitive as well as the most recent or degenerate types being still represented in the sea. But, in a general way, when the geological series is known, it is found to run more or less parallel in time to the progressive changes we must imagine to have taken place. On this fact most recent paleontologists seem to be agreed.
While the phenomena exist and must be reckoned with, the causes are by no means clear. Perhaps the continuous operation of some form of selection may be conceivably potent in some cases. But the more primitive types still retain their vigor and abundance, and this fact must not be overlooked in our explanation. It may be noted that these series, as usually recognized, are made up of genera, each genus a step in some definite direction, with numerous diverging steps at the same time. But there is no evidence that the organisms in question individually vary in any one determinate direction, or that the tide of heredity is swayed by the forces which make for orthogenesis. Most naturalists disclaim any ideological implications in the terms used to describe these phenomena as 'determinate variation.' It is sufficient that it would seem that the line of succession of genera is determined by unknown causes, causes other than those potent in producing the divergence of heredity which we call the origin of species.
We may perhaps find some clue to these matters in the phenomena of analogous variation. Like conditions produce analogous results on forms of very different origin. Osborn notes that nature has often a very limited range of responses to external conditions. In upwards of a dozen different groups of fishes of widely different relationships, nature has developed gar-like jaws. In several different groups (Harriotta, Polyodon, Mitsukurina, Pegasus), she has produced forms with the upper jaw produced into a flat blade. In numerous unrelated groups of fishes, she has produced genera with the parrot's beak (Scarus, Oplegnathus, Tetraodon), or with an imitation of human incisors. In many different wholly unrelated groups she has developed bony plates, hardly to be distinguished superficially from those of her ancient ostracophores and dipnoans. Thus the earlier writers placed with the ganoids such forms as Callichthys, Ostracion, Agonus, Pegasus, Hippocampus, Gasterosteus, Peristedion, forms now known to have no affinities with the extinct mailed fishes, and for the most part no affinities with each other.
Such adaptive characters do not suggest relationship. They are mostly superficial only, and indicate not the origin or affinities of the forms possessing them, but rather the habits of the species in question, and the needs of their recent environment.
In finding what an animal really is, that is, in tracing its ancestry, we have in the words of Haeckel, mainly the three ancestral documents, morphology, embryology and paleontology. Adaptive characters are essentially external. The inside of an animal tells what it is, the outside where its ancestors have been. Perhaps a fuller understanding of orthogenesis will relate its facts to those of 'analogous variation' or the 'convergence of characters' in unrelated forms.
- ↑ The name saltation has been long used for wide fluctuations without recognizable cause. The more recent name mutation chosen by de Vries has been in use for years for the slow changes appearing in geological time.
- ↑ 43, or 11 per cent, of the species found on the Pacific side; about 2.5 of the combined fauna.