2 THE CHARACTERS OF LIVING BEINGS 



particular duty. None are fitted to perform all the functions of 

 life, and none, therefore, can long maintain a separate existence. 

 Thus a skin or muscle cell parted from the rest of the community 

 quickly perishes. Even the duty of continuing the race is dele- 

 gated to a particular set of cells, the germ-cells, which do not 

 otherwise share in the labours of the community. 



2. Germ-cells derived from a female body (e.g. a woman) are 

 termed ova ; whereas those derived from a male body are termed 

 sperms. Sperms and ova differ in appearance, but we have no reason 

 to suppose that they are not equivalent as * bearers of heredity.' 

 They are not male and female ; only the bodies, the cell-communi- 

 ties which they inhabit, are male and female. A sperm from a 

 male body unites with an ovum from a female body. The single 

 cell thus formed is termed the fertilized ovum. The fertilized ovum, 

 dividing and redividing many times, builds up, by means of the 

 descendants thus arising, a new cell-community, a new ' organism,' 

 a new body, a new ' individual,' a human being, for instance. 



3. That fertilization must precede reproduction is a rule almost 

 universal amongst multicellular species. Without fertilization 

 their ova and sperms perish, leaving no descendants. But fertiliza- 

 tion is not quite universal. Some species consist only of females. 

 They are parthenogenetic, that is, the virgin females produce 

 offspring. Without union with another germ-cell, the ovum 

 becomes the ancestor of a new cell-community. More common, 

 especially in the vegetable kingdom, is self-fertilization. Here the 

 individual is both male and female (hermaphrodite) and produces 

 both sperms (pollen grains) and ova (ovules), which unite with one 

 another. But cross-fertilization is by far the most common. It 

 is secured in most animals by the device of sex, in plants more 

 commonly by the ripening at different periods of the sperms and 

 ova of the same plant, or by other devices. Neither cross- nor 

 self-fertilization, then, is necessary to reproduction. But the fact 

 that fertilization, especially cross-fertilization, the mingling of the 

 qualities of two separate cell-communities to secure which nature 

 has evolved sex, is so widespread is evidence that it must possess 

 some very important function, some very important advantage. 

 Darwin found that plants, which normally reproduced by crossing, 

 gave origin to weakly offspring if artificially self-fertilized. It was 

 argued, consequently, that the function of cross-fertilization was to 

 invigorate or rejuvenate. But animals and plants which are 

 parthenogenetic, or which normally fertilize their own ovules, are 

 vigorous. Clearly, then, though crossing is necessary to the well- 



