No. 610] MUTATION THEORY AND SPECIES-CONCEPT 



mediate forms. However these species originated, tliey 

 can not be reasonably supposed to have developed through 

 gradual adaptation, but the color differences probably 

 play no more part in the economy of the species than is 

 the case with the red and gray phases of the screech owls. 

 Something in the germinal organization of Colaptes 

 doubtless determines the definiteness of its color patterns, 

 and it is probable that each element of the pattern was 

 changed by a marked step rather than through a series of 

 gradual stages. This view is strengthened by the fact 

 that a third species, C. chrysoides in Lower California, is 

 essentially a cafer with yellow instead of red quills. 



Thus even in birds our second type of variation, non- 

 adaptational and not related to local conditions, is appar- 

 ently an important factor in speciation, although in 

 Colaptes too geographical races occur as well. In dis- 

 tribution also these species do not follow the rule for 

 geographic subspecies, for they overlap over large areas. 

 The fact that each species has its own geographic sub- 

 species shows that the origin of these species antedates 

 the development of their geographic varieties. 



I have endeavored to show that in plant and animal 

 species there are two distinct types of variability, having 

 different geographical relations. The one is discon- 

 tinuous, independent of environmental or functional in- 

 fluence, and has given rise to many specific and generic 

 characters, notably in plants but also in higher animals. 

 The other is continuous and apparently represents the 

 results of the stress of the environment on the species in 

 its dispersal, leading to the gradual differentiation of 

 local races or subspecies whose peculiarities are ulti- 

 mately intensified and fixed. The latter type of specia- 

 tion is notably exemplified in birds and mammals, organ- 

 isms in which, unlike plants, the individuals can migrate 

 from place to place and so substitute for a stress result- 

 ing from overpopulation an environmental stress caused 

 by a new set of climatic or physiographic conditions. 



