INTEODUCTIOX. 



9 



arbitrary, and that the groups of functions run into one another, and 

 are most intimately connected. Tlius, fur exaniijle, in tlie higher ani- 

 mals, some or all of the functions of relation are secondarily func- 

 tions of nutrition, and vice versa, since the integrity of both groups is 

 essential to the working of either. If the animal be deprived of the 

 powers of vision and locomotion, it cannot obtain food ; and if its 

 nutrition be interfered with, its functions of relation will be destroyed. 



Homology and Axalogy. — These are two terms which are em- 

 ployed to express, respectively, the morphological and physiological 

 likenesses between the organs and parts of different animals. 



When we find the same morphological structure or organ in two 

 different' animals, however much altered or disguised in form, then 

 we have to deal with a case of homoloc/i/ ; the two organs are homol- 

 ogous ; and the one is said to be the Jtomologue of the other. And 



ABC 



Fig. 1.— A, Ann of Man. B, Fore-leg of Dog. C, Wing of Bird, h Humerus, or fione 

 of upper arm ; r and u Radius and Ulna, or b^nes of lite fore-arm; c Carpus, rir 

 bones of tlie wrist ; m Mctacai'pus, or bones of tjn: i-oot of tlie liainl ; p rijalanges, 

 or bones of tlie lingers. 



it is not in the least degTee necessary that the two homologous organs 

 should perform the same fuuctinn- or discharge the same work. On 

 the contrary, they may be fitted for very different purposes, and it 

 is simply necessary that they should tie formed on the same funda- 

 mental plan of structure. For example, the ami of man, the fore- 

 leg of the dog, and the wing of a bird, are constructed upon the 

 same morphological type, and aie therefore /lomo/of/ous — ns will 

 readily be seen liy referring to the annexed illustration, where cor- 



