Page:The New International Encyclopædia 1st ed. v. 14.djvu/325

NATURAL SELECTION. 281 NATURAL SELECTION. tliat all I'xtiiK't beings can lie classed with all recent beings naturally follows from the living and the extinct being the oll'sjiring of common parents. Species have generally diverged in character during their long course of descent and modification, and thus the more ancient types are in some degree intermediate between existing groups. Keeent forms are more imi)roved and generally more specialized than the earlier ones. Vet certain forms have retrograded, ^■hile others have retained "simiilc and little improved struc- tures,' Ix'ing what are called 'persistent types.' (lEOdiiM'iiicAL DisTiilitiTloN. The facts of geographical distribution are also utilized by Darwin, who calls attention to the past migra- tions of animals from one part of the world to another owing to former climatic and geographi- cal changes. They also explain why on the same continent under the most diverse conditions most of the inhabitants within each great class are plainly related, the reason being that they are the descendants of the same progenitors and early colonists. They explain why oceanic islands are inhabited by only few species, most of these, as those of JIadagascar, being peculiar or endemic species. Moreover, the existence of closely allied or representative species in any two areas, as Europe and North America, implies that the same parent forms formerly inhabited both areas. It is also the rule that nearly all the inhabitants of islands have been derived from ancestors which lived on the nearest mainland. Thus the plants and animals of the Galapagos Archipelago, of Juan Fernandez and the other American islands, are closely related to those of the neighboring American mainland, while those of the Cape de Verde Archipelago and other African islands were derived from the opposite African coast. Facts of Morphology, F^MRRyoLooY, the Doc- trine OF HoxtoLOGlES, These were also drawn upon by Darwin. He maintained that adaptive characters, though of paramount importance to the beings, are of hardly any importance in clas- sification, while vestigial characters are often of higli classiflcatory value, the most valuable of all often being embryological characters. "The real allinities of all organic beings, in contradis- tinction to their adaptive resemblances, are due to inheritance or community of descent." N.VTURAL Selection Not the Exclusive Means of ^MoniFiCAXiON. It should be observed that Darwin frankly expressed the conviction that natural selection, though the most important, has not been the exclusive means of modification. He allowed that the alisence of eyes in cave ani- mals is not the result of natural selection, say- ing: "I attribute their loss wholly to di.suse." The Cau.se.s of Variation. In the first edi- tion of his Ynriation. of Animals and Plants Vndrr Ttnmpsticaiion, Darwin, after defining the definite action of the environment, added: "A new sub-variety would thus be produced without the aid of natural selection," but this passage was omitted from the second edi- tion. Besides the Origin nf Sprcics Darwin pub- lished a nundicr of other wru-ks. the most impor- tant of which was The Variation of Animals and Plants under Domestication (1868). Toward the end of his life he gave more attention to the causes of variation. Avliich at first he said were unknown, and in the work just cited he says: "Changrs of any kind in the conditions of life, even extremely slight changes, often suffice to cause variability" (2 ed., ii., ji. 2.j8) ; and again: "Variations of all kinds and degrees are directly or indirectly caused by the conditions of life to which each being, and more especially its an- cestors, have been exposed," adding: "To put the case under another point of view, if it were possible to expose all the individuals of a species during many generations to absolutely uniform coiulitions of life, there would be no variability' lii., pp. 2.53, 255-250). He attril)utes the dif- ferences in races of cats living in Paraguay, at Mombasa. East Africa, and in Antigua, "to the direct action of dillcrent conditions of life" (i., p. 4!)) ; .so with the liorses of the South American pampas and of Puno. He refers to Dr. .J. A. Allen's conclusions relative to the direct action of the climate in producing geographical varieties of birds, and concludes that "these differences must be attributed to the direct action of tem- perature (ii., p. 271). So also, accepting the results of Meehan's comparisons on the leaves of 29 kinds of American trees with their nearest Euroiiean allies, Darwin candidly admits that "such ditl'crence cannot have been gained through natural selection, and must be attributed to the long continued action of a difTerent cli- mate" (ii., p. 271 ) . The objections to his theory raised b}" Darwin himself he discusses with his usual candor. Of these the most important is the absence, to use his own words, of the "interminable number of in- termediate forms," which must have existed, "linking together all the species in each group by gradations as fine as are our present va- rieties." He says he can only answer this objec- tion "on the supposition that the geological record is far more imperfect than most geologists believe." The other objection is the existence of two or three castes of worker or sterile ants in the same community. Status of Darwinism at the Present Day. Such are the views of Darwin, as published in his Oriflin of tSpecies. Although he pushed the special form of evolution to what one would sup- pose to be its furthest limits, yet as we have seen he somewhat modified his views later in life. The views of probably a large proportion of the moderate Darwinians at the present time have been expressed in a broad and can<li<l w;iy by Romanes, an able and careful commentator and expounder of the doctrine of natural selection. In his Dantin and After D<irn-in (1S92), the most clear and readable expose^ of the doctrine, the doctrine is thus stated. All plants and animals are perpetually engaged in the strug- gle for existence. This strife consists in the fact that in every generation of every species a great many more individuals are born than can pos- sibl.v survive. Now nature "selects the best indi- viduals out of each generation to live." ". d not only so, but as these favored individuals transmit their favorable qualities to their oil- spring according to the fixed laws of heredity, it further follows that the individuals composing each successive generation have a general tend- ency to be better suited to their surroundings than were their forefathers. . . . And this follows not merely because in every generation it is only the 'flower of the flock' that is allowed to breed, but also because, if in any generations some new and beneficial qualities happen tn arise as sliglit variations from the ancestral type, the.y will (other things permitting) be seized upon by;