BASIS OF THE PKOBLEM 41 



from the female. The zygote grows and divides and ultimately 

 gives rise to an adult ; into this process of growth it is not necessary 

 to go for the moment. The zygote is to be regarded as a new 

 member of the species the existence of which dates from the 

 fusion of the two nuclei. Whether the zygote lives an independent 

 existence from the beginning, or is retained within the body 

 of the mother for a longer or shorter period, is immaterial so far as 

 the dating of the beginning of the existence of a new individual is 

 concerned. Every man and woman is thus in reahty some nine 

 months older than his or her nominal age. 



4. Such are the essential features of the process of sexual 

 reproduction through which new members of the species arise. 

 We have now to consider the outward features of the process, 

 which vary very considerably. So various are they, that at 

 first sight it may scarcely seem likely that there is any funda- 

 mental generalization that can be made regarding the process 

 as a whole among all species in a state of nature. When we 

 have glanced at the facts and at their interpretation we shall 

 be in a position to discuss what it is that holds good for all such 

 species. 



Among all multicellular animals and plants the number of 

 spermatozoa produced is infinitely greater than the number 

 of ova. A single normal sexual emission in man is said to contain 

 about 226,000,000 spermatozoa. This immense production of 

 male gametes makes it likely that a male gamete wall meet and 

 fuse with each female gamete. Among the higher animals there 

 are certain instincts which further ensure that the male cells 

 will be brought into the proximity of the female cells. Among 

 other multicellular animals and plants there are no such instincts. 

 Generally speaking, among the latter types fertilization may be 

 thought of as fortuitous. In anemophilous plants, for example, 

 such as the Pines, the male cells or pollen-grains are specially 

 adapted so that they are caught and carried by the wind for long 

 distances. Many million times more male cells than female cells 

 are formed in these plants, and some of them, wafted by the 

 wind, eventually light upon the ovule. So too among such 

 lowly animals as the sea-urchin the male and female cells are 

 extruded into the surrounding sea-water ; as these animals live 

 close together, and as the male and female cells ripen and are 

 extruded at the same time, the chance that any egg will remain 



