i8 4 PROBLEMS OF GENETICS 



between those of the two extreme localities, but some third 

 form distinct from either. This is the case for instance in the 

 fauna of brackish waters. We are taught to believe that the 

 fresh water fauna was evolved from the marine fauna, which 

 it well may have been; but as students of Crustacea and Mollusca 

 know familiarly, the brackish water forms are not as a rule inter- 

 mediates between fresh water species and sea species, but more 

 usually they are special forms belonging to the brackish waters, 

 with the peculiar property that they can tolerate a great range of 

 conditions, and live without ostensible variation in waters of 

 most various compositions and densities, which very few marine 

 or fresh water species are able to do. 



Sometimes the distinction between local races, as in Kham- 

 phocoelus passerinii and icteronotus may be regarded with con- 

 fidence as due to one simple Mendelian factor possessed by one 

 race and absent from the other, but I think, more often, as in 

 Colaptes or in the varieties of Pieris napi, the existence of several 

 distinct factors is to be inferred. As we have seen, the races 

 of Colaptes show almost beyond doubt that in different areas at 

 least three distinct factorial combinations can be perpetuated 

 as races. 



In the distribution of variability we find, I think, some hint 

 as to the steps by which the phenomena under consideration 

 have come to their present stage, and I am disposed to regard 

 the facts so well attested in the case of our own melanic moths 

 as a true indication of the process. Following this indication 

 we should regard the change in the character of a population 

 as beginning sporadically, by the appearance of varying indi- 

 viduals, possibly only one varying individual, in, it may be, one 

 place only. As to why a variety should increase in numbers we 

 have nothing but mere speculation to offer, and for the present 

 we must simply recognise the fact that it may. That such sur- 

 vival and replacement may reasonably be taken as an indication 

 that the replacing race has some superior power of holding its 

 own I am quite disposed to admit. Nevertheless it seems in 

 the highest degree unlikely that the outward and perceptible 

 character or characters which we recognise as differentiating the 



