The Origin of Species 329 



mals, and has proved true in many instances, al- 

 though there are numerous exceptions for which 

 as yet there is not an adequate explanation. 



THE EVOLUTION OF SEX 



The characters of the sexual cells in plants and 

 animals often show extraordinary similarity, and the 

 details of fertilization are very much the same. On 

 account of these similarities it is commonly assumed 

 that the nature of sexuality in plants and animals 

 is identical, but a careful examination of the phe- 

 nomena connected with the sexual process in plants 

 and animals will show differences which are suf- 

 ficiently striking to make one cautious in drawing 

 too comprehensive conclusions as to the nature of 

 sexual reproduction in general. 



Evolution of Sex in Plants and Animals Not 

 Identical. Although the nature of the sexual cells 

 in the higher plants and animals is apparently so 

 much alike, the history of the evolution of sex in 

 plants and animals has been apparently quite dif- 

 ferent. In animals, sex is far more important than 

 in plants, and seems to have been established once 

 for all. All Metazoa are sexual at some stage of 

 their development, and there is no existing evidence 

 of that transition from the non-sexual condition to 

 the sexual which is shown so clearly in several evo- 

 lutionary lines in the plant kingdom. Between con- 

 jugation of two individuals found in the Infusoria, 



