22 6 THE EVOLUTION OF SEX. 



the lost epithelium is rapidly replaced, apparently by proliferation from the 

 necks of the glands. By the ninth or tenth day the mucous coat is fully healed, 

 and the beginnings of the next menstrual process recommence. 



The age at which the process commences varies with race and climate, with 

 nutrition and growth, with habit of life (for example, with difference between 

 town and country life), and with mental and moral characteristics. Of these, 

 however, climate seems most important ; thus while in Northern Europe the 

 age is reckoned at the beginning of the fifteenth year, in the tropics it com- 

 mences earlier, in the ninth or tenth year, according to some. The cessation 

 of menstruation usually takes place between the age of forty -five and fifty, and, 

 somewhat as the secondary characteristics of female puberty coincide with its 

 appearance, a less distinct reduction of these is associated with its close ; in 

 many cases secondary resemblances to the masculine type may supervene. 



The old theories of menstruation were, that it served to rid the system of impure 

 blood, that it simply corresponded to the period of "heat" observed in lower 

 animals, or, later, that it was associated with ovulation, — which indeed seems, 

 broadly to correspond with the end of the menstrual period. And while it can 

 not be maintained that either "heat" or ovulation are necessarily associated 

 with menstruation in Homo, there can be little doubt of the general physio- 

 logical parallelism of all three processes. At present there may be said to be 

 two rival theories. According to the first of these, the process is viewed as a 

 kind of surgical "freshening" of the uterus for the reception of the ovum, 

 whereby the latter during the healing process can be attached safely to the 

 uterine wall. The other view is exactly the reverse of this. Its upholders 

 regard the growth of the mucous coat before this commencement of the flow 

 as a preparation for the reception of an ovum if duly fertilized, and the men- 

 strual process itself as the expression of the failure of these preparations, — in 

 short, as a consequence of the non-occurrence of pregnancy. A decided 

 majority of gynaecologists appear to incline to the latter view. 



The process may, however, be expressed in more general, and at the same 

 time more fundamental terms. If the female sex be indeed preponderatingly 

 anabolic, we should expect this to show itself in distinctive functions. Men- 

 struation is one of these, and is interpretable as a means of getting rid of the 

 anabolic surplus, in absence of its consumption by the development of 

 offspring, — just as it is intelligible that the process should stop after fertiliza- 

 tion, when replaced by the demands of the practically parasitic fcetus. In the 

 same way, the occurrence of lactation, after this internal parasitism has been 

 terminated by birth, is seen to be reasonable. The young mammal is thus 

 enabled to become what is practically a temporary' ecto-parasite upon the 

 unfailing maternal anabolic surplus ; and when lactation finally ceases, we have 

 the return of menstruation, from which the whole cycle may start anew. So 

 in the widely different yet deeply similar world of flowers, the distinctly 

 anabolic overflow of nectar ceases at fertilization, and the surplus of continual 

 preponderant anabolism is drafted into the growing seed or fruit. 



IV. Sexual Union. — In a previous chapter we have noted the 

 passive and random way in which the sex-elements of many of the 

 lower animals are liberated, and the chance manner in which they 

 are brought together by water-currents and the like, though this may 

 not be quite so common as our ignorance leads us to suppose, witness 

 the recent observation of the sexual intertwining of Asterina and of 



