Page:Popular Science Monthly Volume 22.djvu/214

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THE POPULAR SCIENCE MONTHLY.

A, B, C, D, E, and F are descended. Even if it could be proved to be the only species of its genus, and therefore the direct ancestor of the recent species, the precise manner in which these are related to it would still remain in uncertainty, for it may have given rise, at about the same time, to two divergent variations, represented by the dotted lines, and these may have led to the two forms a and b, each of which gave rise to variations which resulted in the recent species A, B,

PSM V22 D214 Evolutionary zoology schematic.jpg
Fig. 1.

C, and D, E, F, or it may have persisted for a long time, and become slowly modified, through the stages c, d, e, f, into the form g, different from the form G, and still more different from the known fossil M, before it gave rise to the ancestors of the two recent genera. In this case the phylogenetic tree would be represented by the continuous light lines of the diagram. It is possible, again, that A, B, and C are not equally related, but that two of them, B and C, for instance, have had a common ancestor which was not in the line which led to A, and in this case their relationships would be expressed by the broken lines —.—.—.— of the diagram; or, finally, it is perfectly possible, and many naturalists think it is very probable, that evolution has been guided by some more fundamental law than that of the natural selection of divergent variations, and, if this is so, the characteristics which distinguish A, B, and C may be due to something else