44<) 



THE AMERICAN NATURALIST [Vol. LII 



the aberrant condition, are quite intelligible on the as- 

 sumption that we have to do with a monohybrid recessive 

 character. The character can not be dominant, for we 

 had a case of white-nosed young from two dark-nosed 

 parents. It can not well be due to more than one factor, 

 owing to the relatively large number of recessive indi- 

 viduals. 



VIII. Discussion 



Any one approaching the data dealt with in the fore- 

 going pages, unhampered by theoretical considerations, 

 would, I think, conclude that we had to do with two types 

 of variation and two types of inheritance, differing from 

 one another in rather fundamental ways. In the one 

 class we have the continuously graduated differences, oc- 

 curring within the limits of one of our "subspecies," as 

 well as the differences in average or modal condition 

 which distinguish the various subspecies from one an- 

 other. Here we find a sensible continuity, both within 

 and between these rather artificial assemblages of indi- 

 viduals, and distinct taxonomic units can be recognized 

 only if we erect more or less arbitrary boundaries. In 

 heredity, likewise, we have no indication of a dominance 

 of one step or grade in this series over another, and little 

 to suggest that two of these grades, once united or blended 

 in the offspring, tend to reassert their independence in 

 subsequent generations. 



In the other class we have the "sports" or "muta- 

 tions." These are distinctly discontinuous, in relation to 

 the parent stock, either in the sense that one of the two 

 possesses elements which are altogether lacking in the 

 other, or at least in the sense that the new form has under- 

 gone such a change in the proportions of existing elements 

 that its range of variation does not overlap that of the 

 normal race. Looked at in another light, it is of interest 

 to note that all the mutations which I have discussed, with 

 a single exception, are dependent upon the loss of some- 

 thing. The red-eyed "pallid" mice have lost most of 

 their black and some of their yellow pigment, the "yel- 

 lows" have lost much of their black. The white-tipped 

 tails are due to a loss of part of the dorsal tail-stripe, the 



