2 86 GENERAL BIOLOGY 



5) A species might segregate itself into two groups, if 

 among its members there should arise marked differences as 

 to the date of the breeding season. Those maturing early 

 could only mate with others of like early development, and 

 would thus be segregated, (permanently, if this seasonal 

 habit were heritable) from those that mature late. Differ- 

 ences like this would be likely to be correlated with other 

 differences, and thus two races might begin to diverge. 



6) A species might be segregated into two, if two of its 

 groups of variants should be mutually sterile. Such variants 

 occur among cultivated species. 



7) Two races are developed out of one species when the 

 variants fall apart in two groups, keep together in these 

 groups, develop a "race feeling," and refuse to interbreed. 

 This is reported to have occurred not infrequently when 

 considerable numbers of deer have been kept in private 

 parks. Birds of a feather flock together, even when the 

 feather is distinctive only of a race or a sub-species. This is 

 the kind of segregation known as "preferential mating.'' 



These are the principal means whereby nature keeps her 

 creatures apart in sepaiate strains, or in groups of higher 

 rank; far apart if the barriers be external agencies of isola- 

 tion, but still apart even though near together, if there be 

 such internal agencies as prevent intercrossing. 



The interaction of external and internal forces. So there 

 have been and are still two main types of explanation of the 

 process of evolution, typified by natural selection and 

 orthogenesis; the one emphasizing outward conditions, the 

 other, inner tendencies. The contemplation of the environ- 

 ment, and of the fitness of organisms thereto, leads to the 

 over emphasis of adaptation; the study of the spontaneous 

 and automatic activities of the living substance, tends 

 toward confidence in their sufficiency. The two have much 

 too often been treated as though they were mutually ex- 

 clusive. 



