Il8 MARINE BIOLOGICAL LABORATORY. 



perfect eyes, the eyes of the whole race would begin to 

 degenerate. Take another case, such as is offered by 

 birds of prey, which are possessed of exceedingly sharp 

 sight. If some of these birds happened to come into the 

 world with defective eyes, they would starve to death, or 

 at least be less likely to succeed than their brethren 

 which were more fortunate in this respect. Hence sharp 

 sight is maintained by the continued operation of natural 

 selection, which tends to exterminate those with defec- 

 tive sight. But let these birds come to live under con- 

 ditions in which those with defective sight got along 

 as well as those with keen sight, — say in a dark cave, 

 — and it would seem more than probable that the eye- 

 sight would degenerate. Those which varied in the 

 direction of defective eyes would survive as well as the 

 others. Hence they would breed with the keen-sighted 

 ones, and thus gradually the general average would be 

 lowered. Another example must suffice. Short-sighted- 

 ness is not at all uncommon among civilized races, and 

 is generally believed to be hereditary. Short-sighted 

 men succeed in life as well as those with normal or 

 very keen sight. Neither natural selection nor any 

 other selection compels keen sight. But a short-sighted 

 hawk or antelope, or even a short-sighted Indian, would 

 be placed at so great a disadvantage that short-sighted- 

 ness would soon be eliminated from the race. 



In these last-mentioned forms natural selection com- 

 pels the eyes to come up to a certain standard, while 

 among civilized m^en and cave animals there is no such 

 necessity. Natural selection maintains only what is an 

 advantage, and when it neglects an organ, that organ 

 tends to degenerate. 



