XXIV.] NATURAL SELECTION. 317 



which appeared to be quite peculiar, proving to be really 

 homologous with organs in other species/ that I attach 

 little weight to such an objection. I only mention it as 

 an unsolved difficulty, without implying that it is an 

 insoluble one. If the extensor muscle of the flying 

 lemur's wing-membrane can, for instance, be shown to be 

 homologous with some muscle in any other lemur, similar 

 to those muscles by means of which the horse shakes his 

 skin when tickled, it will be safe to infer that the two 

 muscles are the same in origin, but modified for different 

 work by habitual self-adaptation. 



These wing-membranes — at least in what I believe to be 

 their initial stage — are, as I have already remarked, them- 

 selves very simple organs, and also not in very close 

 correlation with any other organs. But will natural selec- will 

 tion account for changes in organs which are so closely "g^/^'if 

 correlated together that no variation in one of them can be account for 



r. J. J.1 • 1 -J. • -11 closely cor- 



01 any use to the organism unless it is accompanied by related or 

 corresponding variations in all the rest ? And will it complex 

 account for the complex perfection of an organ like the 

 eye or the ear, consisting of many closely correlated parts ? 

 On this subject H. Spencer says : — " The co-operative parts Quotation 

 must vary together, otherwise variation will be detrimental. H°e^ert 

 A stronger muscle must have a stronger bone to resist its Spencer, 

 contractions ; must have stronger correlated muscles and 

 ligaments to secure the neighbouring articulations ; must 

 have larger blood-vessels to bring it supplies ; must have 

 a more massive nerve to bring it stimulus, and some extra 

 development of a nervous centre to supply this extra 

 stimulus. The question arises then, — does spontaneous 

 variation occur simultaneously in all these co-operative 

 parts ? Have we any reason to think that they spontane- 

 ously increase or decrease together ? The assumption that 

 they do seems to me untenable." ^ He goes on to show 

 that if, for instance, the horns of a deer grow larger through 



^ See especially the facts stated in the preceding chapter about respira- 

 tory organs, and those in the chapter on Comparative ^Morphology on the 

 homological relations of the limbs of Vertebrates. 



2 Principles of Biology, p. 453. 



