12 Till: BEGINNINGS OF COMPARATIVE ANATOMY 



profound remark that the possible range of form of an organ 

 is limited to some extent by its existing differentiation. 

 Thus he explains the absence of external (projecting) ears 

 in birds and reptiles by the fact that their skin is hard 

 and does not easily take on the form of an external ear (A 1 

 Prtrtibns, ii., 12). The fact of the inverse correlation is 

 certain ; the explanation is, though very vague, probably 

 correct. 



In one passage of the De Partibns Aristotle clearly 

 enunciates the principle of the division of labour, afterwards 

 emphasised by H. Milne-Edwards. In some insects, he says, 

 the proboscis combines the functions of a tongue and a 

 sting, in others the tongue and the sting are quite separate. 

 " Now it is better," he goes on, " that one and the same 

 instrument shall not be made to serve several dissimilar 

 ends ; but that there shall be one organ to serve as a 

 weapon, which can then be very sharp, and a distinct one 

 to serve as a tongue, which can then be of spongy texture 

 and fit to absorb nutriment. Whenever, therefore, Nature 

 is able to provide two separate instruments for two 

 separate uses, without the one hampering the other, she 

 does so, instead of acting like a coppersmith who for cheapness 

 makes a spit and lampholder in one " (iv., 6, 683 a ). 



(5) The first sentence of the Historia Aniinaliiun 

 formulates, with that simplicity and directness which is so 

 characteristic of Aristotlc.thc distinction between homogeneous 

 and heterogeneous parts, in the mass the distinction between 

 tissues and organs. " Some parts of animals are simple, and 

 these can be divided into like parts, as flesh into pieces of 

 flesh ; others arc compound, and cannot be divided into 

 like parts, as the hand cannot be divided into hands, nor the 

 face into faces. All the compound parts also are made up 

 of simple parts the hand, for example, of flesh and sinew 

 and bone" (( 'rcssvvell, loc. cit., p. i). 



In the DC /\rr/il>/is . Inimaltum he broadens the conception 

 by adding another form of composition. "Now there arc," 

 he says, "three degrees of composition; and of these the 

 first in <>r<lrr, as all will allow, is composition out of what 

 some call the elements, such as earth, air, water, fire. . . . 



