Page:Popular Science Monthly Volume 42.djvu/833

Rh causes in operation, though the matter is presented as though there were.

But passing over this, let us now represent to ourselves in detail this process which Prof. Weismann thinks will, in thousands of generations, effect the observed reduction of the eyes: the process being that at each successive stage in the decrease, there must take place variations in the size of the eye, some larger, some smaller, than the size previously reached, and that in virtue of the economy, those having the smaller will continually survive and propagate, instead of those having the larger. Properly to appreciate this supposition, we must use figures. To give it every advantage we will assume that there have been only two thousand generations, and we will assume that, instead of being reduced to a rudiment, the eye has disappeared altogether. What amounts of variation shall we suppose? If the idea is that the process has operated uniformly on each generation, the implication is that some advantage has been gained by the individuals having the eyes $1/2000$ less in weight; and this will hardly be contended. Not to put the hypothesis at this disadvantage, let us then imagine that there take place, at long intervals, decreasing variations considerable in amount—say $1/20$, once in a hundred generations. This is an interval almost too long to be assumed; but yet if we assume the successive decrements to occur more frequently, and therefore to be smaller, the amount of each becomes too insignificant. If, seeing the small head, we assume that the eyes of the Proteus originally weighed some ten grains each, this would give us, as the amount of the decrement of $1/20$, occurring once in a hundred generations, one grain. Suppose that this eel-shaped amphibian, about a foot long and more than half an inch in diameter, weighs three ounces—a very moderate estimate. In such case the decrement would amount to $1/1440$ of the creature's weight; or, for convenience, let us say that it amounted to $1/1000$ which would allow of the eyes being taken at some fourteen grains each. To this extent, then, each occasional decrement