Page:The American Cyclopædia (1879) Volume VII.djvu/19

EVOLUTION 11 means of Natural Selection;" the other by Alfred Russel Wallace, entitled " On the Ten- dency of Varieties to depart indefinitely from the Original Type." These papers showed that these two naturalists had arrived at almost exactly the same general conclusions ; but the priority may safely be assigned to Darwin, who, although he had not previously made public his views, had submitted a sketch of them as early as 1844 to Sir Charles Lyell, Dr. Hooker, and others. In 1859 he published the treatise entitled " On the Origin of Species by means of Natural Selection," which was the means of diffusing so widely the theory, elab- orated by him through years of patient and careful investigation, that it is commonly des- ignated by his name. In this work he did not apply the doctrine of evolution to the hu- man race, although he had long held the opin- ion that man must be included with other or- ganic beings ; and it was not until after Hux- ley, Spencer, Lyell, Lubbock, Gegenbaur, Vogt, Rolle, Haeckel, Canestrini, Francesco, and others, had accepted the extreme conclu- sion, that he published " The Descent of Man, and Selection in Relation to Sex" (1871). In 1872 Haeckel, who previously had discussed the genealogy of man in Naturliche Schop- fungsgeschichte (1868), published his Mono- graphic der Kalkschwamme, in which he claims to give an analytical demonstration of the prob- lem of the development of species. The the- ory as now generally held is thus stated by Prof. Huxley: "Those who hold the the- ory of evolution (and I am one of them) conceive that there are grounds for believing that the world, with all that is in it, did not come into existence in the condition in which we now see it, nor in anything approaching that condition. On the contrary, they hold that the present conformation and composition of the earth's crust, the distribution of land and water, and the infinitely diversified forms of animals and plants which constitute its present population, are merely the final terms in an immense series of changes which have been brought about, in the course of immeasurable time, by the operation of causes more or less similar to those which are at work at the pres- ent day." The idea expressed by the term development involves the same principle, but it is usually restricted to the evolution of or- ganic beings. We will first consider the doc- trine as applied to the development of the various forms of life, and then in its broader aspects as a theory of universal evolution. It has been proved by geology that the earth and its life, instead of being called suddenly into existence a few thousand years ago, have existed for millions of years ; and as the moun- tains and continents are known to have at- tained their present form by the action of natural agencies, it is thought probable that other objects of nature have been produced in a similar way. The earth has teemed with liv- ing beings through incalculable periods of time, and fossil remains of them are found distributed through the rocky layers that have been suc- cessively formed, until they are several miles in thickness. But not all kinds of animals and plants existed from the beginning, leaving their mingled remains in the lowest strata ; the low- est types of life, vegetable and animal, appeared first. The successive phases of life are so definite that they have been held as mark- ing off the earth's history into a series of ages. The invertebrates (radiates, mollusks, and articulates) are found in the Silurian or oldest stratified rocks ; and from the predomi- nance of the mollusks the period has been called the age of mollusks. Fishes, which are higher in the scale, begin to appear in the Silurian, but become so abundant in the later Devonian period that it is called the age of fishes. Amphibious animals, as an advance on the fishes, appear in the carboniferous age, which again is followed by the age of reptiles. To this succeeds the age of mammals, and last- ly conies the age of man, the series, which be- gan with the lowest forms of life, terminating with the highest. That the order has been progressive, and that its lower terms have been more general in character, while the later terms have been more specialized and perfect, is admitted by all naturalists. Prof. Owen says: "In regard to animal life and its assigned work on this planet, there has plainly been an ascent and a progress in the main ;" and he has " never omitted a proper opportu- nity for impressing the results of observation showing the more generalized structure of ex- tinct as compared with the more specialized forms of recent animals." Prof. Agassiz holds that " the more ancient animals resemble the embryonic forms of existing species ;" that is, are lower in the scale of development than the later forms. Mr. Wallace remarks : " As we go back into past time and meet with the fossil remains of more and more ancient races of ex- tinct animals, we find that many of them are actually intermediate between distinct groups of existing animals;" the ancient fishes, for example, present unmistakable reptilian traits, while the early reptilians combined also the characters of birds which had not yet appeared. As regards the continuity of the course of life, Prof. Dana remarks: "Geological history is like human history in this respect ; time is one in its course, and ah 1 progress one in plan. . . . The germ of the period was long working on- ward in preceding time, before it finally came to its full development and stood forth as ? characteristic of a new era of progress. . . . The beginning of an age will be in the midst of a preceding age ; and the marks of the fu- ture, coming out to view, are to be regarded as prophetic of that future. The age of mammals was foreshadowed by the appearance of mam- mals long before in the course of the reptilian age, and the age of reptiles was prophesied in the types that lived in the earlier carbonif- erous age." The animal kingdom displays a