524 



THE AMEBIC AN NATUBALIST [Vol. LI 



genetic dil¥erences may be. It comes, therefore, some- 

 what as a surprise when Jennings states : 



''Certain serious, difficulties appear in this view of the 

 matter; I shall mention merely two of them, for their 

 practical results. One is the very existence of the minutely 

 differing strains, which forms one of the main founda- 

 tions of the genotype theory. How have these arisen? 

 Xot by large steps, not by saltations, for the differences 

 between the strains go down to the very limits of de- 

 tectability. On the saltation theory, Jordan's view that 

 these things were created separate at the beginning seems 

 the only solution." 



It should be remembered too that it is possible to make 

 up just as continuous a series of characters with genes 

 belonging to different allelomorphic pairs (even when 

 they lie in different linked groups) as the continuous series 

 from multiple allelomorphs. 



If there were any connections between the gradations 

 of character in allelomorphic series and the order in which 

 the characters appear, such a relation might ap])oar to 

 furnish a support to tlu' view that the nssiiined flnctuation 

 of factors is a sequential proco-. and that x'hM-tion actu- 

 ally helps forward tlif direction in which nnitation is 

 likely to take place, a x icw that Ca-tU' lias at times aj)- 

 parently espoused. As a matter df fact, there is no such 

 relation known— the Jmoini fact- are exactly to the con- 

 trary; for the actual evidence from multiple allelomorphs 

 shows that genes may mutate in all directions and also 

 that extreme nmtations such as white eyes arise suddenly 

 from red and not by graded steps 



In the fifth place, the most recent work on Drosophila 

 has shown not only that every m-ne may act (and often 

 does act) as a ditTcretitial for character^ conditioned by 

 other genes, hut al-o that thei-c are -cne- wliose most 

 visible effect i> onlv mi ceitain character- which may 

 therefore 1)e -aid to he modified hy Hi,. f.,rm.M-. It would 

 be a great mistake to >uptio.M' that the,-,. mo,lifying genes 

 are unique in any essential respect the kinds of effects 



