270 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



possibly be shown to be erroneous. The presence of such highly specialized 

 forms, so little changed as to be only specifically separable, at such widely 

 separated localities leads to a consideration of the effect of wide migration 

 upon a fauna. Matthew, in his paper on climate and evolution, discusses 

 this effect. 1 



"Whatever agencies may be assigned as the cause of evolution of a race, it 

 should be at first most progressive at its point of original dispersal, and it will 

 continue this progress at that point in response to whatever stimulus originally 

 caused it and spread out in successive waves of migration, each wave a stage 

 higher than the previous one. At any one time, therefore, the most advanced 

 stages should be nearest the center of dispersal, the most conservative stages 

 farthest from it. It is not in Australia that we should look for the ancestry of 

 man, but in Asia. 



"In the same way, in considering the evidence from extinct species as to 

 the center of dispersal of a race, it has frequently been assumed that the region 

 where the most primitive member of a race has been found should be regarded 

 as the source of the race, although in some instances more advanced species of 

 the same race were living at the same time in other regions. The discovery of 

 very primitive sirenians in Egypt, while at the same time much more advanced 

 sirenians were living in Europe, has been regarded as evidence that Africa was 

 the center of dispersal of this order. It is to my mind good evidence that it was 

 not. It is very common to see references to the African facies of the Miocene 

 or Pliocene mammals of Europe; but it is much more correct to say that the 

 modern African fauna is of Tertiary aspect and is in large part the late Tertiary 

 fauna of the northern world, driven southward by climatic change and the com- 

 petition of higher types. 



"The chief arguments advanced in support of the method here criticized 

 appear to be that the modification of a race is due to the changes in its environ- 

 ment and that the primitive species are altered more and more as they spread 

 out or migrate into a new environment; but, assuming that a species is the product 

 of its environment, the conclusions drawn would only hold true if the environ- 

 ment remained constant. This is assuredly not the case, and if it were there 

 would be no cause left for the species to change its range. In fact, it is the 

 environment itself, biotic as well as physical, that migrates, and the primitive 

 species are those which have followed it, while those which remained have had 

 to adapt themselves to a new environment and become altered thereby. Prob- 

 ably it is never the case that the environment of the marginal species is an 

 absolute replica of the older environment of the race. In many cases it must be 

 profoundly modified by its invasion of new regions, and there are many features 

 in the evolution of a race which appear to be only partly, if at all, dependent on 

 environmental change. But to assume that the present habitat of the most 

 generalized members of a group, or the region where it is now most abundant, 

 is the center from which its migrations took place in former times appears to me 

 wholly illogical and, if applied to the higher animals, as it has been to fishes and 

 invertebrates, it would lead to results absolutely at variance with the known 

 facts of the geologic record." 



1 Matthew, W. D., Climate and Evolution, Annals N. Y. Acad. Sci., vol. xxiv, p. 180, 1915. 



