THEORY OF FERTILISATION. 1 63 



§ 4. Uses of Fer tills atio7i to the Species. — Not a few natu- 

 ralists have passed from the individual aspect of fertilisation to 

 its general import in relation to the life of the species. Why 

 should fertilisation occur at all, if parthenogenesis in some cases 

 works so well ? Part of this question is almost illegitimate, if 

 the existence of male and female be, as we think, simply the 

 expression of a more developed swing of " the organic see- 

 saw " between anabolism and katabolism. The answers have, 

 however, much interest, and are valuable, so long as they are 

 not magnified so as to hide the deeper physiological problems 

 lying below. The origin and physiological import of fertilisa- 

 tion can never be explained by any elucidation of its subsequent 

 advantageousness. 



The two naturalists who have recently reached the most 

 valuable results in regard to the uses of fertilisation are Maupas 

 and Weismann. This they have done by very different paths, — 

 Maupas, in working out the details of conjugation in infusorians ; 

 Weismann, in his wider studies on the problems of heredity and 

 evolution. To Maupas, fertilisation is necessary to prevent the 

 death of the species ; to Weismann, fertilisation is the ever- 

 recurrent beginning of new vital changes, and the continual 

 preservation at the same time of the relative constancy of the 

 species. Several naturalists of the highest reputation have 

 regarded fertilisation as a process which supplied a fresh life- 

 impulse to the species. Thus Galton has insisted, with much 

 clearness and force, on the liability of asexual, or what he calls 

 unisexual multiplication to end in degeneration or extinction, 

 and on the necessity of double parentage for the preservation 

 and progress of the species. Similarly, Van Beneden, Biitschli, 

 and Hensen have all spoken of the process as a rejuvenescence 

 {rejeunissement, Verjiingung). The asexual process of cell- 

 multiplication is limited ; conjugation in lower, fertilisation in 

 higher organisms supply the recurrent impulse which keeps the 

 life of the species young. According to Van Beneden, — " The 

 faculty which cells possess of multiplying by division is limited. 

 There comes a time when they can divide no further, unless 

 they undergo rejuvenescence by fertilisation. In animals and 

 plants, the only cells capable of being rejuvenesced are the 

 eggs ; the only cells capable of rejuvenescing these are the 

 sperms. All the other parts of the individual are devoted to 

 death. Fertilisation is the condition of the continuity of life. 

 Par elle le generateur echappe a la mort." Hensen, in his 



