340 COMPARATIVE ANATOMY 



This principle, brought to light and developed with much skill by 

 H. Milne-Edwards, belongs as much to comparative physiology as to 

 comparative anatomy. It is very general, and is verified among all 

 living beings, both animals and plants. Certain facts are in a way 

 corollaries of it. Thus, organs, even those which are specialized with 

 reference to a given function, may be numerous among lower beings, 

 and have a tendency as soon as and in proportion as an organism 

 perfects itself, to be reduced to a pair, or even to one. Thus the loco- 

 motor members, so numerous in the annelids and myriopods are 

 reduced to four pairs in the arachnids, to three among the insects, 

 to two in the vertebrates, and to one pair in man. 



Principle of the change of functions. This principle, due under this 

 name to A. Dohrn, to whom also we owe the demonstration of numer- 

 ous applications of it, had been clearly formulated by H. Milne- 

 Edwards under the name of the principle of physiological borrowing. 

 He points out that when a new function becomes established, it at 

 first borrows its organs from parts already existing, which change 

 their functions to those to which the modifications correlative with 

 the change have made the organs exactly appropriate. Examples of 

 this are innumerable. One might cite the pectoral fin of the whale, 

 which is only its anterior limb transformed; the poison-gland of the 

 viper, which is a salivary gland; the copulatory appendages of the 

 crab, which are modified abdominal feet. 



It is evident that for the establishment of the majority of these 

 laws comparative anatomy calls upon comparative physiology, and 

 it is even noticeable that the most suggestive laws, those of Milne- 

 Edwards, for example, are more physiological than anatomical. 



There is, however, one last principle which comes wholly from 

 anatomy, and which is proving itself more rich in consequences than 

 all the others united. It may be remarked, in fact, that the number 

 of these general principles is very limited, and it seems as if there were 

 not many more to be discovered. If it were limited, therefore, to the 

 second object, comparative anatomy in having acquired the dignity 

 of a science, properly speaking, could not be considered a very fertile 

 science. 



The principle to which I allude is the following: 



Principle of the uniform constitution of animals. A glance at the 

 organization of beings suffices to show that their structure presents 

 striking resemblances. All mammals have mammary glands and 

 hair, all birds have feathers and wings, all vertebrates have limbs and 

 a bony skeleton, the majority of animals have a stomach, an intestine, 

 a mouth and an anus, muscles for motion, a nervous system to control 

 these muscles, sense-organs often quite comparable, a circulatory 

 apparatus, etc. 



This principle is an idea the acquisition of which goes back to the 



