f,14 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 off 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 natiiral selection to explain 

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

 alleged in 160 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 

 vary 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 



