No. 610] MUTATION THEORY AND SPECIES-CONCEPT 593 



that it is inherited as a simple Mendelian dominant char- 

 acter. The results of the various matings between red 

 and gray would then be as stated above, according to 

 whether the red parent were homozygous or heterozygous, 

 but the ot¥spring from red X red should seldom be all 

 gray, since this would be only a chance result when both 

 parents were heterozygous. It is not impossible, how- 

 ever, since the screech owls usually have only three or 

 four young in a nest, or sometimes only two. 



Since the red phase occurs in various species as well as 

 subspecies it is not improbable that it has originated 

 through independent variations in different species. In 

 any case the geographic ranges of the red phases show 

 that, having appeared as variations, they are inherited 

 without any conspicuous advantage or disadvantage in 

 competition with the gray. The present frequency of the 

 reds in certain areas and their infrequency in others may 

 be merely an indication of the localities where the original 

 mutations took place, and from which as centers they 

 have gradually spread. 



Although the western forms have no red phase, yet 

 Otus asio kennicottii exhibits in addition to its usual 

 tawny-brown phase a relatively rare gray phase. This 

 fact is indeed an argument favoring the assumption that 

 the brown phase of Kennicottii also arose at one step and 

 has since nearly supplanted the original gray form. 



If now we compare the two ij\)Q?> of variability that I 

 have described in Otus asio, we find them sharply con- 

 trasted in several respects: (1) the former is clearly re- 

 lated to geographical distribution, a single race occurring 

 in each locality: the latter has no such relation, but two 

 forms may occur interchangeably in the same place; (2) 

 the former is essentially continuous as a form of vari- 

 ation, the latter markedly discontinuous; (3) the former 

 appears to be related to environmental (climatic) con- 

 ditions, the latter apparently bears no such relation. As 

 regards their evolutionary significance, there can be little 

 doubt that the former or apparently continuous type of 



