Chap. I. HABITS OF ANIMALS. 61 



coiuplR'ated. "Wlu'n inopcrly investigated, especially witliiii the sphere -which con- 

 stitutes more })articularly the essential characteristics of each species of" animals and 

 plants, it is likely to ailord the most direct evidence of the unexpected independence 

 of physical influences of organized beings, if I mistake not the evidence 1 have 

 myself been able to collect. What can there be more chai-acteristic of difterent 

 species of animals than their motions, their plays, their affections, their sexual relar 

 tions, their care of their young, the dependence of these vipon their parents, their 

 instincts, etc., etc. ; and yet there is nothing in all this Avhich depends in the slight- 

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

 they live. Even their organic functions are independent of these conditions to a 

 degree unsuspected, though this is the spliere 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 anatomy and physiology, that 

 identical functions jiresuppose identical organs. Most of our general works upon 

 comparative anatomy are divided into chapters according to this vieAv. And yet 

 there never was a more incorrect principle, leading to more injurious consequences, 

 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 imsound 

 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 the}^ are breathing organs; and yet these gills 

 are formed in a ver}- different way from the lungs ; they bear very different rela- 

 tions to the vascular system ; and it is now known that they may exist simultane- 

 ously with lungs, as in some full-gro■^\^l Batrachians. and, in the earher embryonic 

 stages of development, in all Vertebrata. There can no longer be any doubt now, 

 that they are essentially different organs, and that their functions afibrd no test of 

 their nature and cannot constitute an argument in fovor of their organic identity. 

 The same may be said of tiie vasenlar system of the fi.shes. 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 tlie wann blooded animals; yet embryology has 

 taught us that such a comparison based upcm the special relations of the heart of 

 fishes, is unjustifiable. The air sacs of certain spiders have also lieen considered 

 as lnng.s, because they perform similar respiratory functions, and yvi they are only 

 modified trachea\- which are constructed upon such a peculiar plan, and stand in 



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