244 THE EVOLUTION OF SEX. 



to sexual maturity. A more marked precocity has been ob- 

 served in the x\lpine salamander {Trilon alpestris). In higher 

 organisms, it occasionally happens that long before growth has 

 ceased or adolescence been reached sexuality sets in, especially 

 in the male sex, but this is fortunately a comparatively rare 

 pathological occurrence. In one set of organisms precocious 

 reproductive maturity has been of paramount importance, viz., 

 in the flowering plants. Here the prothallium stage, as con- 

 trasted wnth the vegetative, has been much reduced, and has 

 remained associated with or been absorbed by the asexual 

 generation. This is to be in part explained by the accelerated 

 reproduction of the prothallus, comparable to a similar process 

 which has reduced the separate medusoid sexual persons of a 

 hydroid colony to mere buds. 



§ 2. Menslniation. — The process of menstruation {menses, catamenia), 

 although from the earliest times the subject of medical inquiry, is by no 

 means yet clearly understood. It occurs usually at intervals of a lunar 

 month in all females during their period of potential fertility (fecundity), 

 and so far from being confined to the human species, has been observed at 

 the period of " heat " in a large number of mammals. Though thus clearly 

 a normal physiological process, it yet evidently lies on the borders of 

 pathological change, as is evidenced not only by the pain which so fre- 

 quently accompanies it, and the local and constitutional disorders which so 

 frequently arise in this connection, but by the general systemic disturbance 

 and local histological changes of which the discharge is merely the outward 

 expression and result. In general terms, and apart from ovulation, 

 menstruation may be described as a periodic discharge of blood, glandular 

 secretion, and cellular detritus from the lining of the uteius. After from 

 three to six days the blood ceases to appear, and 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 begin- 

 nings 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 {e.g., 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 commences 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 cannot be maintained that either "heat " or ovula- 



