AGGREGATIONS OF HIGHER ANIMALS 127 



tion in a way that is essentially identical with muta- 

 tion in its mathematical consequences; however, 

 similar results may be obtained in a much shorter 

 time by cross-breeding. And in fact all the different 

 results which have just been illustrated can be dupli- 

 cated by varying the numbers of the emigrants. 



This is not the place to explore all the implica- 

 tions and possibilities of these interesting analyses. 

 The highly significant conclusion has been reached 

 that if a species occurs not as a single breeding unit 

 but broken into effective breeding colonies which 

 are almost isolated from each other, the members of 

 different colonies, given sufficient vigor, may evolve 

 into dissimilar local races. If one of these becomes 

 well adapted to its environment it may increase in 

 numbers and send out numerous emigrants. If these 

 emigrants find and interbreed with members of other 

 less advanced colonies they will grade these up until 

 they resemble the most adapted colony. This part of 

 the process resembles a stock breeder's grading up 

 of a mediocre herd of cattle by repeated infusions of 

 new and improved ''blood" into his herd. The sig- 

 nificant thing here is that the random differentiation 

 of local populations furnishes material for the action 

 of selection on types as wholes, rather than on the 

 mere average adaptive effects of individual genes. 



The end results will vary even when the original 



