464 LIFE : OUTLINES OF GENERAL BIOLOGY 



individuation and genesis vary inversely. Genesis decreases as 

 individuation increases, but not quite so fast; in other words, 

 progressive evolution in the direction of individuation is correlated 

 with a diminishing rate of reproduction. 



Spencer adduced some general physiological reasons why indi- 

 viduation and genesis should vary inversely; he also brought 

 forward inductively a number of instances to show that as a matter 

 of fact poorly individuated types are very prolific, while highly 

 individuated types exhibit economised reproductivity. The tape- 

 worm with its degenerate body and drifting life of ease has its 

 millions of embryos; the Golden Eagle, with its complex body and 

 controlled life, has only two eaglets in a year. There are many facts 

 of this kind showing a coexistence of low individuation and prolific 

 reproduction, of high individuation and small families. The inverse 

 ratio is an observed fact, an outcome of the evolution process. But 

 what Spencer did not prove is that high individuation directly 

 lessens fertility. Perhaps it does, but that has not been proved. 



REPRODUCTION AND HORMONES.— In a higher animal the 

 reproductive maturity is marked by a multiplication and ripening 

 of the characteristic cells in the ovaries and testes (gonads), which 

 often increase in size at the breeding period, and evoke tumescence 

 in their accessory organs. Especially in the male there is a restless 

 urge to secure the relief that the liberation of the reproductive 

 elements brings. But not less important towards all this is the 

 activation of endocrinal tissue associated with the gonads, the 

 result being the production of sex-hormones, which are distributed 

 through the body by the blood. These gonadial hormones stimulate 

 the development of hitherto latent sex-characters, some of which 

 are ancillary to reproduction. Thus the thumb-like swelling up of 

 the first finger in male frogs is of service in the sexual embrace. 

 Similarly the activation of the milk glands in the female mammal 

 prepares for the future suckling of the young; hormones change the 

 character of the uterus for the fixing of the egg-cell and the estab- 

 lishment of the placenta. So, too, the secondary' sexual characters 

 of males, e.g. antlers, manes, beards, etc. There is also in both sexes 

 a marked effect on the nervous system; as at least the liberating 

 stimulus for psychical predispositions previously more or less latent. 

 In highly evolved orgcUiisms like birds as well as mammals, it is 

 impossible to ignore the psychological aspect of reproduction, and 

 there are not a few instances of this in majiy simpler forms as well. 

 HoR.MONiis OF THK Rkprodl'c TiVK ORGANS. — It was from a study 

 of the stimulating effect of injections of testicular extract that 

 lirown-Sequard was led many yrars ago to realise the importance 

 of internal secretions in general; and it is fitting therefore that we 

 should give prominence to the inf}uiry which has developed increas- 



