50 THE FUNCTIONS OF LIFE. 



every other; and hence are deduced what have been termed 

 the laios 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 va- 

 riations in other parts, and extends its influence in modify- 

 ing, 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 occa- 

 sions a change in the centres of gravity, of gyration, and of 

 oscillation; and evolves new mechanical forces and condi- 

 tions of equilibrium, which render new adjustments in other 

 parts necessary, in order to restore the equipoise, and pre- 

 serve the harmony 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, in- 

 vented merely with a view to facilitate the study and to 

 recognise the identity of species, or calculated only to sup- 

 ply 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 king- 

 dom we may trace several points of resemblance, which show 

 them to be parts of one general plan, and to have emanated 

 from the same Creator. In the progress of discovery we 

 are continually meeting with species which occupy interme- 

 diate places between adjacent types, and appear as links of 

 connexion in the chain of being. It often happens, as I 

 shall hereafter have occasion to point out, that throughout 

 an extensive series of organic forms, the steps of gradation 

 by which one type passes into another, are so numerous and 

 so regular, as to preclude the possibility of drawing a de- 

 cided line of demarcation between those that properly ap- 

 pertain to each. " 



