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. One result of failure of the germ to “grow up” into the perfect likeness of its parents is that it may “throw back,” as the breeders say, and resemble in this or that quality a remote, even an extremely remote, ancestor. It is suggested by some inquirers that the congenital or inborn defect, frequent in human beings, which is called “feeble-mindedness” is a reversion or throw-back to the condition of the brain in the animal ancestors of man. That is possible, and, in view of some cases, seems probable. But it must be noted that we do not know what are the causes which favour throwing-back, or “atavism,” as it is called, in regard to all sorts of structures, and that the mechanical conditions connected with the growth of the cavity of the skull in which the brain itself grows are so very elaborate that it is obvious that a very slight disturbance of one element or another might arrest or turn aside the growth of that vastly complex organ, which has become so much larger and more delicate in man than in the animals from which he has, at no remote period in the history of life on the earth, taken his origin.