The Origin of Species 333 



frequently may be entirely suppressed, the sporo- 

 phyte multiplying itself solely by vegetative division. 

 Some seaweeds and many fungi appear to be also 

 entirely sexless, and it is hard to see wherein these 

 plants are inferior in size or structure to the sexual 

 types. This subordination of the sexual conditions 

 in so many plants is in striking contrast with the 

 universal occurrence of sexuality in the Metazoa. 

 It may be safely asserted that the substitution of 

 asexual reproduction for the sexual method in 

 plants is largely due to the generalized character of 

 their tissues, the cells being far more plastic than 

 those of the more highly specialized animals, and 

 therefore capable of an almost unlimited degree of 

 regeneration. 



THE SIGNIFICANCE OF SEX 



The underlying reasons for the development of 

 sex have been the subject of endless speculation, but 

 there is no general agreement among biologists as to 

 what these causes are. Most of these speculations 

 have been from the zoological side, and it is at 

 least doubtful whether they will apply equally to 

 plants, where sexual differentiation might very well 

 have arisen in response to quite different causes. In 

 animals it has been assumed that there is a " physio- 

 logical need " for fertilization, but just what this 

 is, is not explained. Fertilization, or the union of 

 two gametes, has been considered advantagebus both 



