196 Dynamic Theory. 



but still retain their organs, locomotive, digestive, &c. Among the 

 most curious cases of parasitism is that of one sex on the other in the 

 same species. The differentiation by which hermaphrodite or bisexual 

 animals were succeeded by the unisexual, must have been a gradual 

 process, as all natural processes are, and, doubtless, required a great 

 number of generations, and the first results of it must have been ani- 

 mals that differed from each other in no respect whatever, excepting a 

 better development of male glands and a corresponding suppression of 

 female glands in one, and a better development of female and a corres- 

 ponding suppression of male glands in some other. As mentioned be- 

 fore, the Amphioxus, the lowest vertebrate, is in this condition of sex- 

 ual equality. This sexual differentiation is by no means a unique phe- 

 nomenon. It has taken place independently in very many lines of ani- 

 mal development, in various families of Insects, Worms and Mollusks, 

 and also in plants. But subsequent to this primitive equality circum- 

 stances have, in a great many cases, sprung up to create and widen dis- 

 tinction between the sexes. The offices assumed by the sexes are usu- 

 ally complementary of each other, so that each in laying down a func- 

 tion has taken up another, and what is laid down by one is taken up by 

 the other. For example, whilst among monogamous mammals it might 

 be possible for the male to suckle the young equally with the female, 

 among the polygamous ones it is manifestly impracticable as a rule, 

 and so the male mammae are generally rudimentary. But as a rule the 

 polygamous males, in consigning the young to the care of the females, 

 have taken upon themselves the common defense of the family, and 

 anatomical structure following this division of functions, the female has 

 become possessed of the well developed mammae and the broad pelvis, 

 while the male has the heavy shoulders, narrow pelvis and rudimentary 

 mammae. In this, as in all cases of differentiation, the assumption of a 

 new function or the intensifying of an old one, is necessarily accom- 

 panied by and, in some sense, dependent on a corresponding abandon- 

 ment and consequent suppression of another function. Parasitism be- 

 ing always followed by a deterioration of function, the result is not al- 

 tered by a mutuality of parasitism. Everyone loses the power to do 

 for himself that which he habitually puts upon another to do for him, 

 regardless of what he may do in return for that other. In one thing 

 the sexes were at the beginning, and in the very nature of things are 

 always bound to remain, mutualists and equals, that is, in the essential 

 organs and functions of reproduction. But in everything supplemen- 

 tary to this, one sex or the other may assume or shirk out of all duties 

 and correspondingly increase or diminish its structure and function. 

 The female tooth and nail may be sufficient for the family defense, or 

 her industry sufficient to supply the family larder, in which case the 



