114 HEREDITY AND SELECTION IN SOCIOLOGY 



shown plainly by the remarkable cases of hermaphroditism and 

 self-fertilisation. The species whose constitution admits of self- 

 fertilisation have this faculty as an adaptation to the particular 

 conditions of their Ufa. Species such as the Cirripedes, oysters, 

 etc., which are incapable of moving about and going in search 

 of a sexuaUy differentiated partner, have an obvious advantage 

 if they possess the facility of self-fertiUsation, for in this case 

 solitary individuals for which amphimixis would be impossible 

 need not be lost as reproductive members of the species on that 

 account. Hermaphroditism, apart from self-fertilisation, is also 

 advantageous, for did sexual differentiation exist in such seden- 

 tary species, there would be a great risk that two individuals 

 thus differentiated would never come across one another, and 

 thus fertilisation would be impossible. It is, therefore, evident 

 why only hermaphrodite individuals have survived. 



In the Grripedes we find some species not only adapted for 

 self-fertilisation, but at the same time capable of sexual union. 

 The same holds good of certain parasitic worms, which, according 

 as they live alone or in companies within the bladder or intestine 

 of their host, fertilise themselves or exhibit sexual union. In 

 the Cirripedes a male is produced every year, which has the 

 appearance of a rudimentary organism, but is none the less 

 capable of fertiUsing. Later on, the females fertilise themselves, 

 and a second generation is the product of self -fertilisation. But 

 as every year brings a generation of males, amphimixis alter- 

 nates with self-fertiUsation. Some other Crustacea — ^for instance, 

 the parasitic Aniboera — are what is called " protandrous herma- 

 phrodites "; that is to say, they are males in their youth, and 

 become females with the advance of age. 



Darwin noted that, " although various terrestrial species are 

 hermaphrodites, such as the land mollusca and certain earth- 

 worms, all these pair. As yet I have not found a single terrestrial 

 animal which can fertilise itself. This remarkable fact, which 

 offers so strong a contrast with terrestrial plants, is intelUgible 



