16 . ESSAY ON CLASSIFICATION. Pakt L 



the most natural series known in the animal kingdom, every member of which 

 exhibits a distinct grade ^ in the scale ? 



After we have freed ourselves from the mistaken impression that there may be 

 some genetic connection between physical forces and organized bemgs, there remains 

 a vast field of investigation to ascertain the true relations between both, to their full 

 extent, and within their natural limits.^ A mere reference to the mode of breathing 

 of different types of animals, and to their organs of locomotion, which are more 

 particularly concerned in these relations, will remind every naturalist of how great 

 importance in classification is the structure of these parts, and how much better they 

 might be understood in this point of view, Avere the different structures of these 

 organs more extensively studied in their direct reference to the wox'ld in which ani- 

 mals live. If this had been done, we should no longer call by the same common 

 name of legs and wings organs so different as the locomotive appendages of the 

 insects and those of the birds? We should no longer call lungs the breathing 

 cavity of snails, as well as the air pipes of mammalia, birds, and rejDtiles ? A great 

 reform is indeed needed in this part of our science, and no study can prejDare us 

 better for it than the investigation of the mutual dependence of the structure of 

 animals, and the conditions in which they live. 



SECTION III. 



REPETITION OF IDENTICAL TYPES UNDER THE MOST DIVERSIFIED CIRCUMSTANCES. 



As much as the diversity of animals and plants living under identical physical 

 conditions, shows the independence of organized beings from the medium in which 

 they dwell, so far as their origin is concerned, so indej^endent do they aj)pear again 

 from the same influences when we consider the fact that identical types occur every- 

 where upon earth under the most diversified circumstances. If we sum up all these 

 various influences and conditions of existence imder the common appellation of 

 cosmic influences, or of jjhysical causes, or of climate in the widest sense of the 

 word, and then look around us for the extreme differences in that respect upon the 

 whole surface of the globe, we find still the most similar, nay identical types (and I 

 allude here, under the expression of type, to the most diversified acceptations of the 

 word) living normally under their action. There is no structural difference between 

 the herrings of the Arctic, or those of the Temperate zone, or those of the Tropics, 



1 See, below, Sect. 12. ^ g^^^ ^jei^^^.^ gg^t. i6. 



