THE PALEONTOLOGIC RECORD 81 



to prove whether the development of races was or was not wholly con- 

 tinuous. 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. 



THE CONTINUITY OF DEVELOPMENT 



BY DB. T. WAYLAND VAUGHAN 



U. S. GEOLOGICAL SURVEY 



AS 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 nat- 

 ural causes, I will not enter upon a discussion of the truth of the 

 theory of organic evolution, nor will I present the results of phylo- 

 genetic 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 par- 

 ticularly into prominence by de Yries, the theory of saltation, called by 

 him mutation, according to which the progeny differs definitely, with- 

 out intergradation, from the parents, and the difference is perpetuated 

 by heredity. There are two theories of the cause of evolution. Accord- 

 ing 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 transforma- 

 tion and saltation, when superimposed conformable beds contain suffi- 

 ciently abundant faunas, and perhaps the Tertiary marine formations 

 of our southern states will yield important results when studied in 



