70 ESSAY ON CLASSIFICATION 



myself been able to collect. What can there be more characteristic 

 of different species of animals than their motions, their plays, their 

 affections, their sexual relations, their care of their young, the de- 

 pendence of these upon their parents, their instincts, etc., etc.; and 

 yet there is nothing in all this which depends in the slightest degree 

 upon the nature or the influence of the physical conditions in which 

 they live. Even their organic functions are independent of these con- 

 ditions to a degTee unsuspected, though this is the sphere of their 

 existence which exhibits the closest connections with the world 

 around. 



Functions have so long been considered as the test of the character 

 of organs, that it has almost become an axiom in Comparative Anat- 

 omy and Physiology that identical functions presuppose identical 

 organs. Most of our general works upon Comparative Anatomy are 

 divided into chapters according to this view. And yet there never 

 was a more incorrect principle, leading to more injurious conse- 

 quences, more generally adopted. That naturalists should not long 

 ago have repudiated it is the more surprising, as every one must have 

 felt again and again how unsound it is. The organs of respiration 

 and circulation of fishes afford a striking example. How long have 

 not their gills been considered as the equivalent of the lungs of the 

 higher Vertebrata, merely because they are breathing organs; and 

 yet these gills are formed in a very different way from the lungs; 

 they bear very different relations to the vascular system; and it is now 

 known that they may exist simultaneously with lungs, as in some full- 

 grown Batrachians, and, in the earlier embryonic stages of develop- 

 ment in all Vertebrata. There can now no longer be any doubt that 

 they are essentially different organs and that their functions afford 

 no test of their nature and cannot constitute an argument in favor 

 of their organic identity. The same may be said of the vascular sys- 

 tem of the fishes. Cuvier^'^ described their heart as representing the 

 right auricle and the right ventricle, because it propels the blood it 

 contains to the gills, in the same manner as the right ventricle pro- 

 pels the blood to the lungs of the warm-blooded animals; yet Em- 

 bryology has taught us that such a comparison based upon the spe- 

 cial relations of the heart of fishes is unjustifiable. The air sacs of 

 certain spiders have also been considered as lungs because they per- 

 "" Cuvier, Regne animal (2d ed.), II, 122. 



