180 ANNALi^ .V/v'ir YOh'K U'l/)/;i/V OF SCIENCES 



4) Many students of 5j;cogi-ai»liic disLi-ibuLiun i)iiHced on what apiicar to me 

 to be wholly false premises. They assume that the habitat of the most primi- 

 tive living member of a race is the original habitat of the race, the most ad- 

 vanced forms inhal)iting the limit of its migration. It seems to me that we 

 sliould assume directly the reverse of this. 



riMNCIPLES OF DISPERSAL 



Whatever agencies niay be assigned as the cause of evolulioji of a race, 

 it sliould 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 frerpiently 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 mucli 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 competition 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 

 environment 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, tlio conclusions drawn 

 would only hold true if the environment remaiiied 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. 

 Probably, it is never the case thai the cnvironmoit of the marginal 



