CHAPTER XL 



Sexual Reproduction. 



§ I. Different Modes of Reproduction. — It is well known that 

 a starfish deprived of an arm can replace this by a fresh 

 growth ; that crabs can renew the great claws which they have 

 lost in fighting ; and that, even as high up as the lizards, the 

 loss of a leg or a tail can be made good. In a great variety of 

 cases, a kind of physiological forgiveness is shown in the repara- 

 tion of even serious injuries. Now this "regeneration," as it is 

 called, is in a certain degree a process of reproduction. By 

 continuous growth the cells of a persistent stump are able to 

 reproduce the entire member. We know too that a sponge, a 

 hydra, or a sea-anemone, may be cut into pieces, with the result 

 that each fragment grows into a new organism. The same is 

 done with many plants ; and though the division is artificial, 

 the result shows how very far from unique is the process which 

 we usually speak of as reproduction. In fact, as Spencer and 

 Haeckel said long ago, reproduction is but more or less discon- 

 tinuous growth. So again, we pass onwards insensibly from 

 cases of continuous budding, as in sponge or rose-bush, to 

 discontinuous budding in hydra, zoophyte, and tiger-lily, where 

 the offspring, vegetatively produced, are sooner or later set free. 

 Similarly in the Protozoa, an almost mechanical breakage begins 

 the series. This becomes more definite, in the production of 

 several buds at once, or of only one. Budding leads on to 

 deliberate and orderly division, both multiple and binary : 

 while finally, in colonial forms, the liberation of special repro- 

 ductive units may be observed. 



We shall afterwards have to discuss the relations of these 

 and other processes ; but just as we began the study of sex 

 with the familiar contrast of male and female, so we shall begin 

 our investigation of the reproductive processes with the most 

 obtrusive mode, known as sexual reproduction. 



