648 APPENDIX B. 



own, published in the Encyclopcedia Britannica, concerning the 

 production of blind cave-animals. He thinks it can — 



" be fully explained by natural selection acting on congenital fortuitous varia- 

 tions. 5Iany animals are thus born with distorted or defective eyes who^e 

 parents have not had their eyes submiited to any peculiar conditions. Sup- 

 posing a number of some species of Arthropod or Fish to be swept into a 

 cavern or to be carried from less to greater depths in the sea, those individu- 

 als with perfect eyes would follow the glimmer of light and eventually escape 

 to the outer air or the shallower depths, leaving behind those with imperfect 

 eyes to breed in the dark place. A natural selection would thus be effected " 

 in successive generations. 



First of all, I demur to the words " many animals." Under the 

 abnormal conditions of domestication, congenitally defective eyes 

 may be not very uncommon ; but their occurrence under natural 

 conditions is, I fancy, extremely rare. Supposing, however, that 

 in a shoal of young fish, there occur some with eyes seriously 

 defective. What will happen ? Vision is all-important to the 

 young fish, both for obtaining food and for escaping from enemies. 

 This is implied by the immense development of eyes just referred 

 to ; and the obvious conclusion to be drawn is that the partially 

 blind would disappear. Considering that out of the enormous 

 number of young fish hatched with perfect eyes, not one in a 

 hundred reaches maturity, what chance of surviving would there 

 be for those with imperfect eyes ? Inevitably they would be 

 starved or be snapped up. Hence the chances that a matured or 

 partially matured semi-blind fish, or rather two such, male and 

 female, would be swept into a cave and left behind are extremely 

 remote. Still more remote must the chances be in the case of 

 cray-fish. Sheltering themselves as these do under stones, in 

 crevices, and in burrows which they make in the banks, and able 

 quickly to anchor themselves to weeds or sticks by their claws, it 

 seems scarcely supposable that any of them could be carried into 

 a cave by a flood. What, then, is the probability that there will 

 be two nearly blind ones, and that these will be thus carried ? 

 Then, after this first extreme improbability, there comes a second, 

 which we may, I think, rather call an impossibility. How would 

 it be possible for creatures subject to so violent a change of 

 habitat to survive ? Surely death would quickly follow the sub- 

 jection to such utterly unlike conditions and modes of life. The 

 existence of these blind cave- animals can be accounted for only 

 by supposing that their remote ancestors began making excursions 

 into the cave, and, finding it profitable, extended them, genera- 

 tion after generation, further in : undergoing the required adapta- 

 tions little by little.* 



* This supposition I find verified by Mr. A. S. Packard in his elaborate 

 iQonograph on "The Cave Fauna of North America, &c.," as also in his article 

 published in the American Naturalist^ iSeptembcr, 1388; for he there men- 



