290 SEXUAL DIMORPHISM 



and selective survival. We may suppose that the original 

 ancestor had eight arms all alike, and habitually used the 

 third of the left side to convey the spermatophores into the 

 female mantle-cavity. Congenital variations being indepen- 

 dent of the effects of function, we must suppose that all sorts 

 of variations occurred in all the arms, the third left among the 

 rest, and that among all these variations some occurred from 

 time to time which, added together, formed the condition now 

 existing. The objections to this view are, firstly, that the 

 other arms show little or no evidence of the variations which 

 must be assumed, and therefore it is more probable that the 

 required variations only occurred in the arm which was 

 specially employed in copulation ; secondly, on this view the 

 development of the arm in a closed capsule is unexplained, 

 for such a mode of development confers no advantage, could 

 not, so far as we can see, give its possessor any advantage in 

 copulation. On the other hand, if we assume that the 

 modification of the arm, now hereditary and congenital, was 

 originally set up by the mode in which it was used, it seems 

 by no means difficult to discover a correspondence between 

 the muscular action of the arm and its existing structure and 

 development. To trace the relation completely would require 

 a renewed study in detail of the action of the arm in the 

 process of copulation. But it is evident that the spermato- 

 phores are conveyed in a sac formed partly by the bending of 

 the arm at its base towards its dorsal side, partly by mem- 

 branous extensions of the skin of the arm on each side 

 towards the back. Granting that the spermatophores were 

 originally held by the muscular contraction of an unmodified 

 arm, the growth stimulated by these contractions might well 

 lead to the formation of a receptacle. Then, as I have 

 suggested above, the precocious development of the walls of 

 this receptacle and the manner in which the arm was held 

 coiled by muscular contraction, might afford the explanation 





