Page:Popular Science Monthly Volume 77.djvu/484

478 to prove whether the development of races was or was not wholly continuous. But I think that the evidence, considered in relation to the imperfection of our knowledge, goes to show that the gaps were not normally wide. In exceptional cases I think we have reason to believe that they were wide (Otocyon, for instance), but in these instances the evidence is not that of the paleontological record.



S nearly every one now admits the validity of the arguments in favor of the derivations of the existing groups of organisms from previous somewhat different organisms through the operation of natural causes, I will not enter upon a discussion of the truth of the theory of organic evolution, nor will I present the results of phylogenetic studies. We will assume evolution to be true, and having made this assumption, the theories of the process and the underlying causes may be discussed.

Only two theories of the process of evolution seem to me possible: (1) Darwin's theory of gradual transformation, or the origin of new species by the gradual augmentation through successive generations of the difference between progeny and ancestors; (2) that brought particularly into prominence by de Vries, the theory of saltation, called by him mutation, according to which the progeny differs definitely, without intergradation, from the parents, and the difference is perpetuated by heredity. There are two theories of the cause of evolution. According to the first, that of Weismann, the cause is within the organisms themselves, new kinds being produced by an inherent tendency to vary, this tendency being due to differences in the germ cells of the two parents; the second theory attributes the cause to the action of the environment on the organisms inhabiting it.

The fundamental problems of evolution can then be resolved into two questions. Is evolution through gradual divergence from the parental type, or by saltation; and is it caused merely by the differences in the parental germ-plasms or is heritable variation produced by the environment acting on the organisms?

As we are all paleontologists, the question may appropriately be put, what light can paleontology throw on these problems? It may perhaps render some assistance in deciding between gradual transformation and saltation, when superimposed conformable beds contain sufficiently abundant faunas, and perhaps the Tertiary marine formations of our southern states will yield important results when studied in