272 SCIENCE FROM AN EASY CHAIR 



conditions of limited food and space conditions which 

 are not sufficient to support more than a very small 

 number so as to enable them to reach the adult or breed- 

 ing stage of life. But man, on the contrary, protects his 

 own young, and often those of his domesticated animals 

 and plants, from this destructive competition, and thus 

 allows those which are to a certain extent defective to 

 breed. 



The official returns of the Registrar-General record 

 every year a proportion of deaths of infants which are 

 entered as due to " congenital defects." These defects are 

 of many and various kinds. A frequent "congenital 

 defect" is one which does not necessarily cause death, 

 namely, the imperfection of the wonderfully elaborate 

 organ of sight, rendering it useless. Children often are born 

 blind, and so are a certain proportion of each species of 

 animal normally provided with eyes dogs, horses, cattle, 

 birds, fish. Even insects, lobsters, and crabs are in quite 

 considerable and definite number born blind. The inborn 

 or congenital defects of the eyes which result in blindness 

 are of several kinds. The whole of the eyeball or eye- 

 structure may be atrophied, that is to say, dwindled and 

 incomplete, or one essential part only may be defective, 

 and the rest quite well-formed ; or, again, the nerves con- 

 necting the eye with the brain or the several parts of the 

 brain concerned in the function of sight may, one or more 

 of them, be defective. Wild animals born in this condition 

 must perish, except when they happen to be born in 

 caverns or in the deep sea. Then they are no worse off 

 than animals with " good " eyes. But the animals with 

 good eyes, or even with only somewhat defective eyes, 

 will follow up the gleams of light which in moving about 

 they have the fortune to encounter, and so will escape 

 from the cavern or dark depths of the sea, leaving behind 

 in the dark only those with defective eyes. These will 



