INDIRECT EQUILIBRATION. 535 



the formation of different sexes may itself have been deter- 

 mined in the same way. 



To convey here an adequate idea of Mr. Darwin's doctrine, 

 throughout the immense range of its applications, is of course 

 impossible. The few illustrations just given, are intended 

 simply to remind the reader what Mr. Darwin's hypothesis 

 is, and what are the else insoluble problems which it solves 

 for us. 



§ 166. But now, though it seems to me that we are thus 

 supplied with a key to phenomena which are multitudinous 

 and varied beyond all conception; it also seems to me that 

 there is a moiety of the phenomena which this key will not 

 unlock. Mr. Darwin himself recognizes use and disuse of 

 parts, as causes of modifications in organisms; and does this, 

 indeed, to a greater extent than do some who accept his 

 general conclusion. But I conceive that he does not recog- 

 nize them to a sufficient extent. While he shows that the 

 inheritance of changes of structure caused by* changes of 

 function, is utterly insufficient to explain a great mass — 

 probably the greater mass — of morphological phenomena; I 

 think he leaves unconsidered a mass of morphological pheno- 

 mena which are explicable as results of functionally-produced 

 modifications, and are not explicable as results of natural 

 selection. 



By induction, as well as by inference from the hypothesis 

 of natural selection, we know that there exists a balance 

 among the powers of organs which habitually act together — 

 such proportions among them that no one has any consider- 

 able excess of efficiency. We see, for example, that through- 

 out the vascular system there is maintained an equilibrium 

 of the component parts: in some cases, under continued 

 excess of exertion, the heart gives way, and we have enlarge- 

 ment; in other cases the large arteries give way, and we 

 have aneurisms; in other cases the minute blood-vessels give 



way — now bursting, now becoming chronically congested. 

 35 



