FUNCTION. 201 



highest organic types this dependence continues manifest; 

 and it may be traced not only under this most general form, 

 but also under the more special form that in animals having 

 one set of functions developed to more than usual hetero- 

 geneity there is a correspondingly heterogeneous apparatus 

 devoted to them. Thus among birds, which have more varied 

 locomotive powers than mammals, the limbs are more widely 

 differentiated ; while the higher mammals, which rise to more 

 numerous and more involved adjustments of inner to outer 

 relations than birds, have more complex nervous systems. 



58. It is a generalization almost equally obvious with 

 the last, that functions, like structures, arise by progressive 

 differentiations. Just as an organ is first an indefinite rudi- 

 ment, having nothing but some most general characteristic 

 in common with the form it is ultimately to take; so a 

 function begins as a kind of action that is like the kind of 

 action it will eventually become, only in a very vague way. 

 And in functional development., as in structural development, 

 the leading trait thus early manifested is followed succes- 

 sively by traits of less and less importance. This holds 

 equally throughout the ascending grades of organisms and 

 throughout the stages of each organism. Let us look at 

 cases: confining our attention to animals, in which func- 

 tional development is better displayed than in plants. 



The first differentiation established separates the two 

 fundamentally-opposed functions above named the accumu- 

 lation of energy and the expenditure of energy. Passing over 

 the Protozoa (among which, however, such tribes as present 

 fixed distributions of parts show us substantially the same 

 tiling), and commencing with the lowest Ccelenterata, where 

 definite tissues make their appearance,, we observe that the 

 only large functional distinction is between the endoderm, 

 which absorbs nutriment, and the ectoderm which, by its 

 own contractions and those of the tentacles it bears, produces 

 motion: the contractility being however to some extent 



