(THE FEEBLE-MINDED 273 



breed together, and perpetuate their defective eyes in a 

 more and more marked degree in successive generations. 

 Always those which can see, or see only a little, will leave 

 the dark place, and so at last there will be a race of 

 animals established in such places with defective eyes (as 

 in the ocean, at a depth of two thousand fathoms, and in the 

 great caves of Europe, and notably in the Mammoth Cave 

 of Kentucky, U.S.A.), and often the eyes will altogether 

 disappear. This, however, is a digression. 



The cause of congenital defect in eyes is not obvious. 

 The failure of the germinal substance of the reproductive 

 egg or particle detached from parents with sound eyes, to 

 unfold itself to develop into a creature with sound eyes 

 like those of its parents, is apparently due, in some 

 instances, to one cause, and in other instances to other 

 causes, of none of which can it be said that we have a 

 satisfactory and comprehensive knowledge. The same 

 want of full knowledge exists in regard to the causation 

 of other congenital defects. These are as numerous and 

 varied as the parts of the body which may be from birth 

 onwards, in one instance or another, distorted, devoid of 

 some essential inner structure, swollen to great size, or 

 shrunken, or even absent altogether. Such defects are 

 sometimes caused by mechanical pressure or nipping in 

 early stages of growth before birth, but there are many 

 which cannot be accounted for in that way. The wonder 

 really is not that the inconceivably complex structure of 

 higher animals should sometimes fail, in this or that part, 

 to develop in due course from the simple-looking germ, 

 destitute of visible structure, but that it should, on the 

 contrary, so regularly and completely come off successfully 

 in millions of instances every day. We can imagine or 

 suggest disturbing agencies which may be the causes of 

 failure, but it is very difficult to demonstrate with certainty 

 the causes which have been at work in each and every case. 



