No. 601] MENDELIAN FACTOR DIFFERENCES 33 



to insist that the factors can not be regarded as deter- 

 miners in themselves; but rather that they are differenti- 

 ators, that working together with other factors in the 

 system a difference is produced in somatogenesis which 

 has its origin in some difference, some change in a locus 

 in the system. For when in Drosophila a change in the 

 locus W is produced, or in Y, such that the individuals 

 developing from sj^stems with these changed loci are 

 white-eyed in the one case and yellow-bodied in the other, 

 it seems evident that the change is more profound than 

 the color of eyes or of body; that beyond these changes 

 there is an underljang, elusive, physiological change re- 

 sulting in individuals that are less vigorous and less fer- 

 tile than those which develop from the normal unchanged 

 system. The fact that a factor may have a primary, 

 simple, easily recognizable effect and secondary far reach- 

 ing effects, the latter to be attributed to the modified 

 physiological relations resulting from a change in one of 

 the members of a system, is one which has often been ob- 

 served and which is of fundamental significance in our 

 conception of the interrelations of the genetic factors. 

 There are, however, other instances which may be cited 

 of a somewhat similar nature, locus changes which pro- 

 duce certain characteristic effects under particular en- 

 vironmental conditions, but fail to disturb the normal 

 behavior of the factor system when these conditions are 

 not met. 



It would, of course, be possible to recount almost in- 

 definitely the specific effects of particular locus changes, 

 whereas evidence concerning these far reaching effects of 

 single factors is too scanty to warrant further discussion 

 of this point. However, Morgan (1915 a) has been able to 

 establish the relations displayed by the factor for ab- 

 normal abdomen and to demonstrate that only under very 

 particular conditions is the presence of this genetic factor 

 manifested by its characteristic expression, and that when 

 these conditions are not present the product of somato- 

 genesis may not differ in appearance from the noraial fly, 

 although differing from it both in genetic constitution and 



