INTRODUCTION. 



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

 are most intimately connected. Thus, for example, in the higher ani- 

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

 tions of nutrition, and vice versd, 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. 



HoMOLOQY AND ANALOGY. — 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 homology ; the two organs are homol- 

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



ABC 



Fig. 1.— A, Arm of Man. B, Fore-leg of Dog. C, Wing of Bird, A Humerus, or bone 

 of upper arm ; r and u Radius and Ulna, or bones of the fore-arm ; c Carpus, or 

 bones of the wrist ; m Metacarpus, or bones of the root of the hand ; p Phalanges, 

 or bones of the fingers. 



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

 should perform the same function or discharge the same work. On 

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

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

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

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

 same morphological type, and are therefore homologous — as will 

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



