238 HABIT AKD INTELLIGENCE. [cHAP. 



brought under tlie law of adaptation. It is only Ungulates 

 with an even number of toes on the foot — that is to say, 

 cloven-footed animals — which have horns on each side of 

 the head. The only Ungulate with an odd number of toes 

 Rhino- that has a horn is the rhinoceros ; and, instead of a horn 

 *'®™®' on each side of the head, he has one on its middle line, on 

 the snout. This coincidence is certainly remarkable. I 

 do not, however, lay much stress on it, because the peculiar 

 position of the horn of the rhinoceros may be an adapta- 

 tion to his muscular structure, enabling him to use his 

 weapon to the best advantage. But it is at least equally 

 probable that his muscular structure is adapted to the 

 position of the horn, and that the position of the horn is a 

 case of correlation between the head and the feet, and is 

 no more due to any adaptation of structure to function 

 than are the formative laws of crystallization. 

 Resem- It is Very generally tlie case that different parts of the 



of different Same organism, even when not homologous, are formed in 

 parts in much the same way. In other words, both the tissues and 



the same .,.,,. , . , . 



organism, the manner m which the tissues are combmed into organs, 

 are alike in the different parts of the same organism. Thus, 



Verte- bone is peculiar to the Vertebrata, and is found both in the 

 spiiie and in the limbs ; and the bones of both the spine 



Artieulata. and the limbs are covered by the muscles. In the Articu- 

 lata, which in many respects form a contrast and a kind of 

 inverted resemblance to the Vertebrata, the body is divided 

 into segments, and the limbs also are jointed ; and in both 

 the body and the limbs the hard parts cover the muscles, 

 instead of being covered by them : so that the type of 

 their construction is opposite to that of similar parts in 



MoUusca. the Vertebrata. In the Mollusca there is no skeleton at 

 all, with the ^single exception of the cartilaginous skull of 

 the cuttle-fish tribe (for the shell cannot be regarded as a 

 skeleton) ; the body is not segmented, and there are no 

 jointed limbs, only soft tentacles. Yet this is not con- 

 nected with any inferiority of organization, for the organs 

 of vegetative life among the Mollusca are very highly 

 organized ; and the Cephalopoda (cuttle-fish and nautUus), 

 which are the highest Molluscan class, stand as high in 



