INBREEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION 239 



yet in man}^ cases gonochorism has been relinquished, and always 

 where this was necessitated by the conditions of life to wliich the 

 group concerned was subject. In such a case inbreeding is regulated 

 as far as possible, for instance, by an arrangement which ensures 

 that individuals shall be crossed at least from time to time. But 

 cases of exclusive and constant self-fertilization do also seem to 

 occur, and even these may be brouglit into harmony with our 

 conception, according to which cross-fertilization is an advantage, 

 but only an advantage which must be weighed against others, and 

 which may eventually be given up in favour of greater advantages. 

 This occurrence of persistent autogamy can no more be reconciled 

 with the rejuvenation theory than can continuous parthenogenesis, 

 because, according to this theory, the mingling of different individuals 

 is a sine qua non for the continued life of the species. 



It is impossible for me here to discuss in detail all the deviations 

 from pure gonochorism or bi-sexuality which occur in nature, but 

 I must at least attempt to take a general survey, and to arrange 

 the chief phenomena of these various modes of ' sexual reproduction ' 

 in an orderly scheme. I must take a survey of both plants and 

 animals, but I shall give the precedence to animals, as being to me 

 more familiar ground. 



Where do we find, in the animal kingdom, that Nature has 

 departed from gonochorism, from the separation of the sexes, and 

 for what reasons was this departure necessary ? And further, what 

 means does Nature take to compensate for this renunciation of the 

 simplest method of securing the continual cross-fertilization of 

 individuals ? 



Let us glance over the animal kingdom Avith special reference 

 to these questions : we find that hermaphroditism prevails chiefly 

 among species which at maturity have lost their power of free 

 locomotion, and have become sedentary, such as oysters, barnacles 

 among Crustaceans, the Bryozoa, and the sea-squirts (Ascidians) 

 which are fixed to the rocks at the bottom of the sea. For forms 

 such as these it must often have been advantageous that each 

 individual could function both as male and as female, especially 

 when it was capable of self-fertilization, since individuals whicli 

 settled down singly, or in very small numbers together, would not 

 be lost as regards the persistence of the species. The continuance 

 of the species is thus better secured than it would be by separation 

 of the sexes, because in the latter case it might frequently have 

 happened that the animals which had settled beside each other by 

 chance were of the same sex, and would therefore remain unfertile. 



