'284 AV. D. MATTHEW PHYLOGENY AND CORRELATION 



Their application to zoological, and especially to paleontological, studies 

 is, I believe, no less exact, but has received little attention. 



Admixture of Blood through Migrations 



In presenting the theory of dispersal in its application to fossil verte- 

 brates, it is convenient to speak of a series of waves of migration proceed- 

 ing outward from a center of dispersal. In fact this is a somewhat mis- 

 leading phrase. What must really have occurred is rather a more or less 

 continuous stream of new blood pouring into the region considered, mix- 

 ing with the older blood and gradually displacing it. If the course of 

 migration be uninterrupted and the environmental pressure be more or 

 less continuous in its action and direction, the progress of a race in a 

 given region will be due partly to the infusion of new blood from the 

 adjoining region nearer the center of dispersal, partly to the adaptive in- 

 fluence of the local environment, partly to the substitution of new species 

 or genera arriving from regions nearer the center of dispersal and which 

 either were originally or have become infertile with the autochthonous 

 species. 



In discussion of the evolution of a phylum, the second and fourth of 

 these influences have usually been considered. But it would seem that all 

 four have frequently, if not generally, played an important part in deter- 

 mining the evolutionary progress of a phylum in any locality. The rep- 

 resentatives of a phylum at any one stage in its development in a given 

 region do not consist of one or several homogeneous species or pure races, 

 but of series ranging from archaic autochthones to recent immigrants, 

 more or less broken into nodes or separated groups by local segregation 

 or the partial extinction of intermediate members of the series. 



The English Eace as an Illustration 



Let us consider, first, what we know to have been occurring in the mi- 

 grations of the human race and its effect on the population of Great 

 Britain. When this island first appears in history its population is some- 

 what heterogeneous. The inhabitants of the south and east are closely 

 allied to the Gauls. Northward are traces of Scandinavian and in Ire- 

 land of Iberian affinities. The Roman occupation, while it checks migra- 

 tion for a time, does not introduce any important new ethnic element, but 

 toward its close the normal course of migration is resumed and an ever 

 increasing flood of Platt-Deutsch peoples pours in. The effect of the 

 hindrance of migratory movements during Roman times is seen in a par- 

 tial disinclination to free admixture, and a certain degree of segregation 



