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Page:Popular Science Monthly Volume 19.djvu/235

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things at the close of the Cretaceous age, and exhibiting little or no change during their relatively brief history.

Such cases of stability amid conditions which might well have favored change, and which saw copious modification and progression in other groups of animals, might at first sight be regarded as presenting a serious obstacle to the doctrine of progressive development on which the whole theory of evolution depends. As such an obstacle, the series of facts in question was long regarded; as such, these facts are sometimes even now advanced, but only by those who imperfectly appreciate and only partially understand what the doctrine of evolution teaches and what its leading idea includes. Even Cuvier himself, when advancing the case of the apparently unchanged mummies of Egyptian animals against Lamarck's doctrine of descent, failed—possibly through the imperfectly discussed stage in which the whole question rested in his day—to understand that the very facts of preservation revealed in the monuments of Egypt testified to the absence of those physical changes which could alone have affected the animals of the Nile land. But the fuller consideration of that theory of nature which credits progressive change as the usual way of life, shows us that it is no part of evolution to maintain either that living beings must needs undergo continual change, or that they must change and modify at the same rate. On the contrary, Mr. Darwin, in his classic work, maintains exactly the opposite proposition. There are, in fact, two great factors at work in living nature—a tendency to vary and change, and the influence of environments or surroundings. Given the first tendency, which is not at all a matter of dispute, the influence of the second is plainly enough discernible in bringing to the front either the original,

PSM V19 D235 Ichthyosaurus and plesiosaurus.jpg
Figs. 6 and 7.—Ichthyosaurus and Plesiosaurus.

primitive, or, as it might be named, the parent form, or the varying forms which are produced by modification of the parent. As it has well been put: "Granting the existence of the tendency to the production of variations, then, whether the variations which are produced