Page:Popular Science Monthly Volume 22.djvu/213

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cover, and the circumstantial evidence converges in such a way as to give in this case every assurance of substantial accuracy.

There is not a single wild species of animal which can be traced, by direct unbroken pedigree, to an ancestor belonging to a different genus, and no zoölogist has any hope of ever obtaining anything more than circumstantial evidence of such a pedigree; but we can hardly overestimate the vast and increasing stores of evidence in favor of evolution which are yielded by the structural, geographical, and chronological relations between the fauna of the present day and that of the past. It is true that all this evidence is circumstantial, and, although it renders the theory of evolution vastly more probable than any other explanation of the origin of species, it still leaves a possibility that some other explanation may be the true one. Although the investigator who is fully acquainted with all the evidence may feel justified in ignoring this possibility, it still exists.

If the evidence which we have is so circumstantial that it does not amount to absolute proof, it is clear that, even if we fully believe in evolution, we can not hope to trace, with anything like minute accuracy, the past history of any particular form of life; but perhaps an illustration may help to make this clearer:

Let the dots A, B, and C (Fig. 1), represent a number of recent species, each of which has distinctive characteristics of its own, together with other characteristics which are common to all; and let D, E, and F be another set of species related to each other in the same way; and suppose that certain of the common characteristics of D, E, and F are also common to A, B, and C, while others distinguish the one set as a whole from the other as a whole. According to the theory of evolution, we believe that A, B, and C are the descendants of an ancestor from whom they inherit all that they have in common; and that D, E, and F are related to each other in the same way; that the common ancestor of all the forms in the first group had, together with distinctive characteristics of its own, certain other characteristics which it shared with the ancestor of the forms in the second group, and that this similarity was due to inheritance from a still more remote ancestor common to both. This system of relationship might be expressed by a phylogenetic tree, like that which is shown by dotted lines. . . . in the diagram, with six ultimate ramules, two large branches, and a common stem, G. Now, suppose that we discover, in a recent geological formation, a fossil form, M, which resembles A, B, C, D, E, and F in all the features which they have in common. It is possible that this fossil is the form G, from which A, B, C, D, E, and F are descended, but it is not probable that this is the case, for the analogy of recent species compels us to believe that the fossil M was one of several closely related species, G, H, I, K, and L, any one of which may have been the ancestor of the recent forms, and, as M is only one out of several species, the chances are that it is not the root G from which