PROFESSOR WEISMANN'S THEORIES. 487 



with a body and head attached ; and if he then remembers that 

 every egg contains material for such a pair of eyes ; he will see 

 that eye-material constitutes a very considerable part of the fish's 

 roe ; and that, since the female fish provides this quantity every 

 year, it can not be expensive. My argument against Weismann 

 is strengthened rather than weakened by contemplation of these 

 facts. 



Prof. Lankester asks my attention to a hypothesis of his own, 

 published in the Encyclopaedia Britannica, concerning the pro- 

 duction of blind cave-animals. He thinks it can 



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

 tions. Many animals are thus born with distorted or defective eyes whose 

 parents have not had their eyes submitted to any peculiar conditions. Supposing 

 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 individuals 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 genera- 

 tions. 



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, how- 

 ever, 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. 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 crayfish. 

 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 subjection to 

 such utterly unlike conditions and modes of life. The existence 



