614 APPENDIX B. 



underground streams it inhabits are unusually swollen, some in- 

 dividuals of the species are carried out of the caverns into the 

 open (being then sometimes captured). It is also said that the 

 creatures shun the light ; this trait being, I presume, observed 

 when it is in captivity. Now obviously, among individuals 

 carried out into the open, those which remain visible are apt to 

 be carried oif by enemies ; whereas, those which, appreciating the 

 difference between light and darkness, shelter themselves in dark 

 places, survive. Hence the tendency of natural selection is to 

 prevent the decrease of the eyes beyond that point at which they 

 can distinguish between light and darkness. Thus the apparent 

 anomaly is explained. 



Let me suggest, as another possible reason for persistence of 

 rudimentary organs, that the principle of economy of growth will 

 cause diminution of them only in proportion as their constituents 

 are of value for other uses in the organism ; and that in many 

 cases their constituents are practically valueless. Hence prob- 

 ably the reason why, in the case of stalk-eyed crustaceans, the 

 eye is gone but the pedicle remains, or to use Mr. Darwin's simile, 

 the telescope has disappeared but not its stand. 



Along with that inadequacy of natural selection to explain 

 changes of structure which do not aid life in important ways, 

 alleged in § 166 of The Principles of Biology, a further inadequacy 

 was alleged. It was contended that the relative powers of co- 

 operative parts cannot be adjusted solely by survival of the 

 fittest ; and especially where the parts are numerous and the co- 

 operation complex. In illustration it was pointed out that im- 

 mensely developed horns, such as those of the extinct Irish elk, 

 weighing over a hundred-weight, could not, with the massive 

 skull bearing them, be carried at the extremity of the out- 

 stretched neck without many and great modifications of adjacent 

 bones and muscles of the neck and thorax ; and that without 

 strengthening of the fore-legs, too, there would be failure alike 

 in fighting and in locomotion. And it was argued that while we 

 cannot assume spontaneous increase of all these parts propor- 

 tionate to the additional strains, we cannot suppose them to in- 

 crease by variations, one at once, without supposing the creature 

 to be disadvantaged by the weight and nutrition of parts that 

 were for the time useless — parts, moreover, which would revert 

 to their original sizes before the other needful variations occurred. 



When, in reply to me, it was contended that co-operative parts 

 vaty together, I named facts conflicting with this assertion — the 

 fact that the blind cray-fish of the Kentucky caves have lost 

 their eyes but not the foot stalks carrying them ; the fact that 

 the normal proportion between tongue and beak in certain 



