INTRODUCTORY ii 



generation. Again in other animals, such as the bee, partial 

 parthenogenesis occurs, the female giving rise to two kinds of 

 eggs, one kind developing without fertilisation and giving rise 

 to drones or males, while the eggs of the other kind, which are 

 fertilised by stored-up spermatozoa, develop into workers and 

 queens, both of which are females. 



Hermaphroditism. — In some animals which propagate sexually, 

 instead of the ova and spermatozoa being produced by two sexual 

 individuals, they are formed by the same individual, which is 

 then said to be hermaphrodite or monoecious, the bisexual con- 

 dition being described as dioecious. In certain hermaphrodite 

 species the two sorts of sex cells are produced simultaneously 

 by the same individuals, but in these the spermatozoa and ova 

 are formed at different seasons, the individual animals sometimes 

 functioning as males and sometimes as females. We shall return 

 to this subject later in considering the phenomenon of sex 

 determination. 



With hermaphrodite animals in which the ova and spermatozoa 

 are produced simultaneously self-fertilisation sometimes occurs, 

 but cross-fertilisation (or the union of eggs and sperms coming 

 from different individuals) is far commoner. Thus Morgan has 

 shown that with the ascidian, Cynthia partita, although self- 

 fertilisation occurs, the eggs are far more frequently fertilised 

 by sperms from separate individuals. In another ascidian, 

 Ciona intestinalis, self-fertilisation is only occasional. Both 

 these animals are marine and the gametes are passed out into the 

 sea-water. Amongst plants, as Darwin showed, very ingenious 

 contrivances exist to ensure cross-fertilisation, and this method is 

 the more usual in both the vegetable and the animal kingdom. 

 Inbreeding, which implies the mating of animals which are of near 

 kin, may be regarded as similar to self -fertilisation, except that 

 the conjugating cells are one or more degrees distant in relation- 

 ship, and it is noteworthy that under some conditions at any rate 

 close inbreeding may result in eventual sterility. As already 

 mentioned the deterioration resulting from close inbreeding 

 has been held to be comparable to that taking place among some 

 Protozoa when conjugation does not occur, and that in either 

 case change of environment may prevent the adverse conse- 

 quences of the lack of " fresh blood." This is a matter which we 

 shall refer to again in a later chapter. 



