274 THE EVOLUTION THEORY 



stances, to enemies, or to lack of food, the more prolific must the 

 species be. The same holds true of the regulation of the number of 

 male germ-cells to be produced by an individual ; there must be so 

 many developed that, in spite of the unavoidable enormous loss, on an 

 average the number of mature ova necessary to the maintenance of the 

 species always receive spermatozoa. 



Also associated with the prodigal production of zoosperms is their 

 minuteness, for the more zoosperms that can be developed out of 

 a given mass of organic substance the smaller they are. Each species 

 is restricted within definite limits of production by its size and the 

 volume of its body, and there is thus an advantage in having the 

 zoosperms of the smallest possible size whenever the chance of the 

 individual sperm successfully reaching an ovum is very small. In 

 all such cases nature has abstained from burdening the male germ- 

 cell with an appreciable contribution of material to the result of 

 conjugation, that is, to the foundation of the new organism ; the 

 passive ovum contains in itself alone almost all that is necessary to 

 the building up of the embryo. Fertilization of the ovum by the 

 liberation of the sperm-cells into the water occurs not only in animals 

 of low degree, such as Sponges, Medusae, Star-fishes, Sea-urchins and 

 their relatives, but also in much higher animals, such as many Fishes 

 and Amphibians, and in these the male cells have the form of motile 

 threads. But the spermatozoon-form of male cell does not occur 

 only in animals and plants which live in the water, or in those which, 

 like mosses and many vascular plants, are at least occasionally covered 

 by a thin layer of rain or dew, in which the zoosperms can swim to 

 the ova, it occurs also in a very large number of animals in which 

 the sperms pass 'directly into the body of the female, in those, 

 therefore, in which copulation takes place. 



But even where copulation occurs we find that in most cases, as, 

 for instance, in Vertebrates, Molluscs, and Insects, the zoosperm-form 

 is retained. The reason for this is obviously twofold : in the first 

 place, in many cases the sperms do not directly reach the ovum as 

 a consequence of copulation, but may have to go a long way within 

 the body of the female, as in mammals ; or even when the way is short 

 and certain, the ovum may be encased in a firm covering or shell 

 difficult to penetrate, and the thread-like zoosperm has to face the 

 task of boring its way through this shell, or slipping in through 

 a minute opening, the so-called micropyle. In either case it would 

 be difficult to imagine a form of sperm-cell better adapted to the 

 fulfilment of this task than that of a thread with a thin, pointed head- 

 portion and a long motile tail, which enables the zoosperm to twist 



