THE FUNCTIONS OF LIFE. 51 



laws of the co- existence of organic forms. By 

 attention to these laws we may often explain how 

 each variation that is observed in any one organ, 

 common to a natural group of animals, entails 

 certain necessary and corresponding variations 

 in other parts, and extends its influence in modi- 

 fying, in a greater or less degree, the whole fabric. 

 It is in comparative anatomy as in mechanics, 

 where any alteration made in the position of one 

 part of a system of bodies occasions a change in 

 the centres of gravity, of gyration, and of oscilla- 

 tion ; and evolves new mechanical forces and 

 conditions of equilibrium, which render new 

 adjustments in other parts necessary, in order 

 to restore the equipoise, and preserve the har- 

 mony of their movements. 



We may conclude from these inquiries that 

 the numerous classes or assemblages of beings, 

 which science has formed, are by no means 

 arbitrary creations of the human mind, invented 

 merely with a view to facilitate the study and 

 to recognise the identity of species, or calculated 

 only to supply the imperfections of our memory ; 

 but that they have a real foundation in nature. 

 To regard any of the beings in the creation as 

 isolated from the rest, would be to take a very 

 narrow and a false view of their condition ; for 

 all are connected by mutual relations. Even 

 among the leading types which represent the 

 great divisions of the animal kingdom we may 



