Page:The American Cyclopædia (1879) Volume VII.djvu/20

 12 EVOLUTION unity of plan or a correlation of parts by which common principles are traced through the most disguising diversities of form, so that in aspect, structure, and functions the various tribes of animals pass into each other by slight and gradual transitions. The arm of a man, the fore limb of a quadruped, the wing of a bird, and the fin of a fish are homologous ; that is, they contain the same essential parts modified in cor- respondence with the different circumstances of the animal ; and so with the other organs. Prof. Cope says : " Every individual of every species of a given branch of the animal king- dom is composed of elements common to all, and the differences which are so radical in the higher grades are but the modifications of the same elemental parts." There are many cases of rudimentary and useless organs in animals and plants. During the development of em- bryos organs often develop to certain points, and are then reabsorbed without performing any function, although generally the partially developed organs are retained through life. Certain snakes have rudimentary hind legs hidden beneath the skin ; the paddle of the seal has toes that still bear external nails; some of the smooth-skinned amphibia have scales buried under the dermal surface ; rudi- mental teeth have been traced even in birds ; and there are rudimentary eyes in cave fishes and rudimentary mammae in men. Classifica- tion is an arrangement of living beings by re- lated characters. In the earliest attempts the organic tribes were arranged in a serial order or a chain from the bottom to the top of the scale ; but this has been abandoned, as also have those symmetrical systems which as- sumed that the characters of different groups are equivalents of each other. The endeavor to thrust animals and plants into these arti- ficial partitions is of the same nature as the endeavor to arrange them in a linear series ; and it assumes a regularity which does not exist in nature. Classification now represents the animal kingdom as consisting of certain great sub-kingdoms very widely divergent, each made up of classes much less widely divergent, severally containing orders still less divergent, and so on with genera and species, like the branches of a growing tree ; and the old meth- od of classification, as Mr. Spencer remarks, involves exactly the difficulty " which would meet the endeavor to classify the branches of a tree as branches of the first, second, third, fourth, and fifth orders ; the difficulty, namely, that branches of intermediate degrees of com- position exist." There is a remarkable analogy between the present distribution of animals in space over the earth and their past distribution in time as we trace their fossils in the succes- sive geological formations. The larger groups, such as classes and orders, are generally spread over the whole earth, while smaller groups, such as families and genera, are commonly con- fined to limited districts ; but when a group is restricted to one region, and is rich in the minor groups called species, it is almost in- variably the case that the most closely allied species are found in the same locality or in closely adjoining localities. The same fact is seen in geological distribution. Mr. "Wallace observes : " Most of the larger and some smaller groups extend through several geologi- cal periods. In each period, however, there are peculiar groups, found nowhere else, and extending through one or several formations. As generally in geography no species or genus occurs in two very distant localities without being also found in intermediate places, so in geology the life of a species or genus has not been interrupted. In other words, no group or species has come into existence twice." From these facts Mr. Wallace deduces the following important law : "Every species has come into existence coincident both in space and time with a preexisting closely allied species." The adherents of development maintain that these facts, and many others of kindred significance, are only to be explained by the continuous operation of a great natural law of descent and divergence by which the present life of the earth has been derived from its preexisting life. That the numberless forms of life should have been held as independently created, so long as the earth was regarded as having been recently and suddenly called into existence, was inevitable; but now, when it is known that the order of nature is extended backward into immeasurable time, the supposition that species were called into existence by hundreds of thousands of separate and special creations, running through the geological ages, and as we approach our own epoch suddenly and un- accountably ceasing, is held to be an unwar- ranted assumption which science can no longer accept. As remarked by the Rev. Baden Powell : " The introduction of a new species is part of a series. But a series indicates a principle of regularity and law, as much in organic as in inorganic changes. The event is part of a regularly ordained mechanism of the evolution of the existing world out of former conditions, and as much subject to regular laws as any changes now taking place. If the series be regular, its subordinate links must each be so ; the part cannot be less subject to law than the whole. That species should be subject to exactly the same general laws of structure, growth, nutrition, and all other functions of organic life, and yet in the single instance of their mode of birth or origin should constitute exceptions to all physical law, is an incon- gruity so preposterous that no inductive mind can for a moment entertain it." This is the ground taken by the great majority of contem- porary naturalists. They believe in evolution in some form as a great fact of nature ; but many think that we know nothing as to how it has been brought about, while others hold that the problem of the modes and causes of evolution, although obscure, is no more barred from successful investigation than are the other