266 HABIT AND INTELLIGENCE. [cHAP. 



the species only, these are monstrosities ; but considered 

 with reference to the type of the class, I have no doubt 

 they are cases of reversion to it. 

 Funda- The case of the flounders is a good instance of the 



^l^tive^ difference between fundamental and adaptive characters, 

 characters. Embryonic characters, as we have seen, are fundamental. 

 On the development theory of the origin of species, they 

 are inherited from a very remote ancestry ; and to the 

 •class of fundamental characters belong those homological 

 resemblances, mentioned in the last chapter, between 

 organs that are unlike in external form and in function, 

 but present a minute resemblance in structure which 

 cannot be accounted for by the law of adaptation of 

 structure to function : such as the parallelism of formation 

 between the hand of the man, the fore-foot of the quad- 

 ruped, the wing of the bat, and the paddle of the whale. 

 Adaptive characters, on the contrary, are acquired later in 

 the course of development, and consist in modifications 

 of the fundamental ones, so as to adapt them to the 

 peculiar mode of life of the organism : such as the 

 peculiar modifications of the common type, which fit the 

 hand, the foot, the wing, and the paddle, each to its 

 Homology special function. Homologous parts are those which are 

 *'^^, developed in the same way, and their resemblance is 



consequently fundamental. "When organs are analogous 

 without being homologous, on the contrary, such as 

 the wing of the bat and the wing of the insect, they 

 are developed in different ways, and have no resem- 

 blance in their embryonic states. Their resemblance is 

 acquired later, and is not fundamental, but only adaptive. 

 Homo- To put the contrast in its sharpest form — when organs 

 refem- ^^® homologous without being analogous, the resemblance 

 blances ig fundamental, and the difference is adaptive ; and the 

 mental, resemblance is greatest at first. Such are the man's hand 

 and the bat's wing. When organs, on the contrary, are 

 analogous without being homologous, the resemblance is 



both eyes are on the same side : " The eye changes little in actual position : 

 with the growth of the fish the associated parts are, as it were, developed 

 past it, producing this singular obliquity." 



