478 THE POPULAR SCIENCE MONTHLY 
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 Dr. T. WAYLAND VAUGHAN 
U. S. GEOLOGICAL SURVEY 
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 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 Vries, 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 
