﻿GATES— PAIRS OF SPECIES 



of M. bifolium; and Actaea rubra var. gigantea is probably a cell 

 giant of A . rubra, from which it has apparently arisen by a mutation. 

 In the case of Clintonia borealis and C. umbellulata , the peculiarities 

 of the latter probably represent a differential of three or four 

 definite and independent variations. In this way would arise a 

 series of forms, all of which have been extinguished except the two 

 remaining. This hypothesis differs from the Darwinian theory of 

 natural selection only in assuming that the inherited variations are 

 usually not infinitesimal, but bold and definite strokes. We are 

 merely applying the conceptions gained from the facts of experi- 

 mental breeding. 



The pair Sire pto pus amplexifolius and 5. roseus presents a 

 similar problem. There are four main pairs of character differences 

 between these species. They may be assumed to have arisen 

 through a series of mutations from a common ancestor. Inter- 

 crossing would lead to various combinations and in some cases 

 blends of these mutant characters. Many such combinations 

 would be gradually eliminated through their own instability or 

 their sterility in producing offspring, leaving finally the present pair 

 of species as survivors. The differences between Ranunculus aborti- 

 vus and R. allegheniensis are such that the latter, which is more 

 limited in its distribution, may be reasonably assumed to have 

 arisen from the former through a single positive mutation. It is 

 very difficult, if not impossible, to believe that the conspicuous beak 

 of the achene, which is the main peculiarity of R. allegheniensis, 

 could have been developed gradually through natural selection. 

 It is much more probable that the character has no selective value 

 and is merely inherited because it has appeared as a germinal 



In Actaea, after a somewhat detailed analysis of the differences 

 between A. rubra and A. alba, it was found desirable to consider 

 the whole genus as it now stands, and incidentally three new 

 varieties were described. The differences between A. rubra and 

 A. alba are much more numerous than might have been anticipated, 

 yet two mutations are perhaps sufficient to account for the origin of 

 the latter from the former. The thickening of the pedicels in A . alba 

 was found to be due to the fact that the rows of cortical cells are 



