18 ESSAY ON CLASSIFICATION 



organized beings, there remains a vast field of investigation to ascer- 

 tain 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, were the different structures of these organs more ex- 

 tensively studied in their direct reference to the world 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 loco- 

 motive 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 reptiles! A great reform is indeed 

 needed in this part of our science, and no study can prepare us bet- 

 ter 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 iden- 

 tical physical conditions shows the independence of organized beings 

 from the medium in which they dwell, so far as their origin is con- 

 cerned, so independent do they appear again from the same influ- 

 ences when we consider the fact that identical types occur everywhere 

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

 all these various influences and conditions of existence under the 

 common appellation of cosmic influences, or of physical 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 ac- 

 ceptations of the word) living normally under their action. There is 



^ See below, Sect. xvi. 



