286 filGH SCHOOL zoologV. 



incapable of adapting themselves to any changes. On the other 

 hand, we may explain to ourselves the persistence of cei'tain 

 types (VIII, 25), either by the little specialised character of their 

 requirements, or by their gaining superior means of protec- 

 tion, as for example, the adoption of a subterranean life, by 

 terrestrial animals, or their retirement to the recesses of deep 

 forests. 



The competition of different species is interesting in another 

 aspect — the regidation of the balance of life. It is obvious that 

 the number of carnivorous animals is regulated by the number 

 of phytophagous forms on which they feed, and that these, 

 again, are dependent on their food-plants. 



23. While reference is being made to species which live in 

 competition with each other, it juust be recalled that many in- 

 stances of association for mutual advantage are to be found in 

 Nature. One of the most striking of these is afforded by some 

 Sea-anemonies, which fasten themselves over the abdominal 

 region of certain Paguridse. They serve in place of the shelter- 

 ing shells, which most of the genera select (VII., 12), and, in 

 addition, protect the crab by reason of their thread-cells ; in 

 return, they are furnished with locomotive facilities not usually 

 enjoyed by their relations. Other examples of such mutualism, 

 or symbiosis, are not uncommon. 



24. Still more common, however, are the c ses of partial or 

 complete parasitism, which ai-e to be met with in all the sub- 

 kingdoms. (See index). The various grades of parasitism 

 offer such easy transitions from completely normal to much re- 

 duced organs, that we are tempted to seek an explanation for 

 these in the direct action of the environment. It is especially 

 the locomotive and digestive oi'gans which exhibit such reduc- 

 tion, and the disuse of these seems to offer a I'ational explan- 

 ation of their condition. 



25. In the course of dLsa2)pearance of organs which are under- 



