v] LATER PROGRESS 127 



from symbiosis to parasitism, though the host here 

 enters the relation of its own free will. Convoluta 

 somewhat resembles an employer of slave-labour in a 

 country where slaves are very kindly treated : the 

 green slaves are well provided for during their in- 

 dividual lives, but they have sacrificed the power of 

 further perpetuating their species. A growing Con- 

 voluta plus its contained green cells is therefore that 

 anomaly, a temporary individual. 



Take next a case of true parasitism. With most 

 internal parasites, such as trypanosomes (the flagel- 

 lates which cause sleeping-sickness and other diseases), 

 or tapeworms, each species of parasite is confined 

 normally to one host-species, and cannot come to 

 perfection elsewhere. It is often extraordinarily 

 closely adapted to its environment both in structure 

 and life-history, as a study of any tapeworm will 

 show ; but that environment is an extremely limited 

 one. 



After this consider an apparently very remote 

 subject the relation between insects and flowers. 

 I need here merely point out that many insects, such as 

 bees and butterflies, procure all their food from the 

 honey or pollen of flowers, and that most plants with 

 conspicuous flowers rely exclusively or chiefly on in- 

 sects for their fertilization, and so for their continuance 

 as species. Both insect and flower have been radically 

 modified in structure and appearance through this 



