i895. THE ROLE OF SEX. 199 



the muscle cells, and yet they are the largest. We are driven back, 

 therefore, to our first statement, which appears to contain all the 

 facts of universal application at present known, namely, that there 

 are gametes which are larger, contain capital, and are approached 

 by the others — these are termed female ; and there are those which 

 are smaller, which actively approach, or are passively carried to, the 

 ova — these we term male. 



Dimorphism of the Individual. — Having indicated the obvious 

 advantages which result from dimorphism of the reproductive cells, 

 let us now turn to a consideration of the individual as a whole. 



Volvox produces both motile — male — and large quiescent — female 

 — sexual cells. But Volvox A is like Volvox B, every specimen produc- 

 ing both male and female cells. Volvox as an individual has no sex, 

 strictly speaking. Qua reproductive cell there is sex difference — 

 dimorphism ; qua individual Volvox there is no such difference. 



In by far the greater number of higher forms the male cells are 

 carried by some, the female cells by other, individuals ; there is a 

 division of labour, and here qua individual we may introduce the 

 terms male and female : there is individual dimorphism. In most 

 cases the two individuals are modified on different lines, and this 

 modification is, we find, always associated with some useful purpose. 

 Organs are specially developed to bring about the more complete 

 mixing of the reproductive elements, other organs to aid the nutrition 

 and development of the young, other organs for protection or active 

 aggression. Division of labour, an obvious economy, necessitates 

 the possession of the organs of nutrition by one individual — the 

 female — and of organs of defence by the other — the male. 



These sexual differences, as we term them, vary both in kind 

 and degree. Except by dissection, we cannot distinguish those fish 

 whose sole functional difierence is, in one case, to produce ova and 

 discharge them in the sea or on the river bottom, in the other, to 

 produce sperms and discharge these over the ova. The male stickle- 

 back, on the other hand, with his multifarious duties of builder and 

 guardian, is distinguishable at once from his mate ; and the sexes in 

 the rays and dogfish are very distinct, for here, again, important and 

 different duties are undertaken by each of their sexes. The starfish, 

 which produces sperms, is, both in external and internal structure, 

 similar to the starfish which produces eggs, for in this class the 

 duties of maternity and paternity are usually no more than to grow 

 their reproductive cells and simply discharge them into the sea where 

 they conjugate and develop. In the mammal, where the ova develop 

 within the mother, important structural differences exist between the 

 sexes ; the mother becomes a nurturing individual, and the father a 

 protecting one. 



If we now turn to a study of dimorphism of the individual, with 

 a view of determining whether or not there are any characters 

 common to all males, or whether there are any characters common to 



