5 1 6 Studies in the Theory of Descent. 



It is not inconceivable although a clear proof 

 of this is certainly as yet wanting that a variety 

 of the ancestral species would differ only in one 

 single character, such as hairiness, colour, or 

 marking, and such instances would thus agree 

 precisely with the foregoing cases in which only 

 the caterpillar or the butterfly formed a variety. 

 All the more profound modifications however 

 such for instance as those which determine the dif- 

 ference between two species are never limited to 

 one character, but always affect several, this being 

 explicable by correlation, which, as Darwin has 

 shown in the case of dogs, may cause modifications 

 in the skull of those breeds having hanging ears in 

 consequence of this last character alone. It must 

 be admitted however that one organ only would 

 be originally affected by a modifying influence. 

 Thus, I am acquainted with two species of a 

 genus of Daphniacea which are so closely allied 

 that they can only be distinguished from one 

 another by a close comparison of individual de- 

 tails. But whilst most of the external and in- 

 ternal organs are almost identical in the two 

 species the sperm-cells of the males differ in a 

 most striking manner, in one species resembling 

 an Australian boomerang in form and in the other 

 being spherical ! An analogous instance is fur- 

 nished by DaphniaPulsx and D. Magna, two species 

 which were for a long time confounded. ' Nearly 

 all the parts of the body are here exactly alike, 



