230 



HABIT AND INTELLIGENCE. 



[chap. 



Acantlio- 

 metrse. 



Correla- 

 tions in all 

 organisms 

 not refer- 

 alile to 

 adapta- 

 tion. 



Classes of 

 homo- 

 logies. 



forms of the Acanthomdrce, whicli are low animal organisias 

 not very remote from the Foraminifera. These appear to 

 consist of nothing but structureless sarcode, with a skeleton 

 of siliceous spicules ; they are allied to the Thalassicolliclce, 

 of which, perhaps, they ought to be regarded as a genus. 

 But the Acanthometrce are very much more definite in 

 form than most of the Thalassicolliclce; yet this definite- 

 ness of form does not appear to be accompanied by any 

 corresponding differentiation of function between different 

 parts — or, in other words, by any physiological division of 

 labour ; and, so far as I can see, the beautiful regularity 

 and symmetry of their radiated forms are altogether due to 

 unknown laws of symmetry of growth, just like the equally 

 beautiful and somewhat similar forms of the compound 

 six-rayed, star-shaped crystals of snow. 



The adaptation of structure to function is the law, and 

 indeed the definition, of organization ; and the higher 

 we ascend in the organic scale, the more definite and 

 complete is this adaptation. But even in the very highest 

 forms there are homologies and correlations which cannot 

 be regarded as simply instances of adaptation. Before 

 stating any of these, it is necessary to explain with more 

 definiteness what is meant by homological relations. There 

 are homologies between different species, and there are 

 homologies within the limits of the same species. I shall 

 consider first those which are within the limits of the 

 same species. 



Of such homologies there are four distinct cases : — 



1. Between different parts of the same organism ; 



2. Between the same organism at different stages of 



its development ; 



3. Between the sexes of the same species ; and 



4. Between forms that mutually produce each other 



by metagenesis. 

 The first three cases occur among all organisms what- 



beauty. ... In the same drop of moisture there may be some dozen or 

 twenty forms, each with its own distinctive pattern, yet aU as constant as 

 they are distinctive, yet having all the same habits, and without any 

 perceptible difference of function." 



