THE EVOLUTION OF SEX 45 



with its asexual, hermaphrodite, unisexual, 

 and parthenogenetic forms, is a very epitome 

 of the evolution of sex. 



TESTES AND OVARIES. In a sponge, which 

 shows no organs in the strict sense, and only 

 the beginnings of tissues, the germ-cells appear 

 sporadically in the middle stratum (or meso- 

 gloea) of the body, and both sperms and ova 

 may be seen developing in the same sponge. 

 An important step was taken, however, when 

 the formation of gametes began to be localised 

 and when these foci of gamete-making began 

 to be enclosed, protected, and nourished by 

 adjacent tissue. At first, doubtless, the differ- 

 ence between the egg-multiplying gonads (or 

 ovaries) and the sperm-multiplying gonads 

 (or testes) was only to be detected with the 

 microscope, though it was very profound. 

 But gradually there came about a specialisa- 

 tion of the accessory parts in relation to the 

 deeply different functions of egg-multiplying 

 and sperm-multiplying, and recognisably dif- 

 ferent ovaries and testes were established. 

 Furthermore, there came to be associated with 

 ovaries and testes a good deal of auxiliary 

 tissue that had nothing directly to do with 

 oogenesis or spermatogenesis. Accessory 

 glands and ducts were formed. 



It is probable that variations or mutations 

 involving the tissue in the vicinity of the 

 reproductive organs have been from time to 

 time caught up into their service, and it is 

 not necessary to suppose that these advan- 

 tageous accessions, such as having a yolk- 

 gland near the ovary, arose as pure coinci- 

 dences. For it is a well-known fact (whether 



