J. E. DUERDEN 189 



of any character increases the probability of further stages in the same 

 direction.... In this sense evolution is more likely to take place along the 

 lines already followed if further advantage is to be found in that direc- 

 tion." It is contended that the facts disclosed by the ostrich show that 

 the principle of sequential, cumulative evolution holds quite apart from 

 the last clause ; " advantage " has nothing whatever to do with the 

 changes nor with the direction which they follow. 



The claw on the fourth toe is perhaps the simplest, most straight- 

 forward example we can select from among the many structural parts 

 which are undergoing degeneration, and discussion of some of the genetic 

 principles involved may well centre around it. It may be allowed that 

 a continuous series of stages can be found between its maximum de- 

 velopment and its last trace in the integument before its final disappear- 

 ance, the smallness of the steps between one stage and the other 

 depending upon the number of claws available, a theoretically large 

 enough number giving a theoretical continuity. But what are the fac- 

 torial issues involved ? Have we a single germinal factor or even group 

 of factors which is undergoing as slow and continuous a weakening, or 

 is it a successive loss of factors each of which has such a small expres- 

 sive power that apparent continuity prevails ? The question can be 

 stated in several other forms : Is the individual factor itself capable of 

 variability, of changing its potency up and down, as it were, in a con- 

 tinuous degree ; or are the variations in its expressibility dependent 

 upon a successive loss of the constituent factors of a main factor 

 (multiple allelomorphs) ; or is the factor itself immutable and degene- 

 ration the result of other factorial influences acting upon it, the latter 

 being of a modifying, controlling, or inhibitory nature (multiple 

 modifying factors), and possibly arising as-a fractionation of a complex 

 factor ? 



Discussion of the various aspects of the problem of factorial varia- 

 bility is most acute at the present time. The controversy centres mainly 

 around the work of Prof W. E. Castle on hooded rats and that of 

 Prof T. H. Morgan and his associates on the fi'uit-fly Drosophila. The 

 issues are plainly stated by Prof H. S. Jennings, as follows^ : " Castle 

 finds that in rats he can, by selection, gradually increase or decrease the 

 amount of colour in the coat, passing by continuous stages from one 

 extreme to the other. As to this, he holds two main points : 



"1. The change is an actual change in the hereditary characteristics 



1 "Observed hanges in Hereditary Cliaracters in relation to Evolution," Jour. Wash. 

 Acad. Sciences, Vol. vii. 1917, p. 287. 



