244 



HABIT AND INTELLIGENCE. 



[chap. 



Homolo- 



gical re- 

 semblances 

 carried 

 further 

 tliau ueces- 

 sary. 



Homolo- 

 gical 

 resem- 

 blances 

 and adap- 

 tive dif- 

 ferences. 



It cannot in the least surprise us that organs, or sets of 

 organs, like the pectoral and ventral fius, the caudal fin, 

 and the dorsal fin, which are in no degree homologous as 

 to their relation to the rest of the body, should be adapted 

 for the same work of swimming, and for that purpose have 

 received the same structure ; being all formed of skin, 

 supported and extended on a framework of bony rays. 

 This is only a case of that adaptation which is the purpose 

 of organization, and which we expect to find everywhere. 

 But what really needs explanation is, that homological 

 resemblances are in many cases carried much further than 

 is needed for any adaptation to purpose. We have seen in 

 the last chapter that this is the case within the limits of 

 the same species ; as, for instance, between the hands and 

 the feet of the human species, and between the two sexes 

 of the same. It is equally true, that between different 

 species and groups there are homological resemblances 

 of a degree and a kind for which no law of adaptation 

 will account, though they are wonderfully harmonized 

 with the law of adaptation. This is most evident among 

 the Vertebrata, the general plan of which is much more 

 constant than that of any of the lower groups, while it is 

 equally su.sceptible of adaptive modifications for all pos- 

 sible kinds of life. This wonderful combination and 

 harmony between homological resemblances and adaptive 

 differences is best seen in tracing the homologies of the 

 skeleton. 



What I mean by the combination of homological resem- 

 blances with adaptive differences may, if the expression is 

 not of itseK intelligible, be understood by comparing the 

 hands and the feet of man, which have the strongest 

 homological resemblance, and yet are adapted for different 

 functions — the feet for walking, and the hands for grasping. 



Vertebrata, being the side where the nervous centres are. Huxley calls 

 the side of the body which contains the nervous centres the neural side ; 

 that which contains the circulatory centre, or heart, the haemal side. In 

 the Vertebrata the neural side is the back, and the haemal side the beUy : 

 in the Mollusca and Articulata, their relative positions are reversed. 

 (Huxley ou the Morphology of the CVphalous Mollusca, Philosophical 

 Transactions, 1853.) 



