Page:Popular Science Monthly Volume 71.djvu/208

202 of almost, but not quite nine per cent., rapidly falling down so that after the chick is two months old it never adds as much as three per cent, to its weight. It loses in the first two months from a capacity to add nine per cent., down to a capacity of adding less than three. It loses in two months two thirds of its total power of growth, for from nine to zero is divisible into two parts, of which the first, from nine down to three, would be two thirds, and the second, from three to zero, would be one third. Here then we learn that two thirds of the decline which occurs in the life of a chick takes place in two months, and for the rest of the life of the bird there is a decline of one third. That, you must acknowledge, is an extraordinary and most impressive difference. If it be true that the more rapid growth depends upon the youth of the individual,—its small distance in time from its procreation, then we may perhaps, by turning to other animals which are born in a more immature state, get some further insight into these changes; and that I have attempted to do by my observations upon the development of rabbits. Rabbits, as you know, are born in an exceedingly immature slate. They are blind, they are naked, they are almost incapable of definite movements, quite incapable of locomotion, and are hardly more than little imperfect creatures lying in the nest and dependent utterly upon the care of the mother, quite unable to do anything for themselves except take the milk which is their nourishment. They are indeed animals born in a much less advanced stage than are the guinea pigs. Upon the screen we see this interesting result demonstrated to us, that a male rabbit, the fourth day after its birth, is able to add over seventeen per cent, to its weight in one day. From that the curve drops down, as you see, with amazing rapidity, so that here at an age of twenty-three days the rabbit is no longer able to add nearly eighteen per cent, daily, but only a little over six. At the end of two months from its birth, the growth power of the rabbit has dropped to less than two per cent., and at two months and a half it has dropped to one. The drop in two and a half months has been from nearly eighteen per cent, down to one per cent., and the rest of the loss of one per cent, is extended over the remaining growing period of the rabbit. Could we have a more definite and certain demonstration of the fact that the decline is most rapid in the young, most slow in the old? It is not in this case any more than in the others the one sex that demonstrates this fact, for in the female we find exactly the same phenomena, as the next slide will show. The irregularities are not significant. The strange dip at thirty-eight days, for instance, corresponds to an illness of some of the rabbits which were measured, but they rapidly recovered from it and grew up to be fine, nice rabbits. If instead of measuring half a dozen rabbits, we had measured two hundred or five hundred, these irregularities would certainly have disappeared. The females in the case of the rabbits, as in the case of the guinea-pigs, are not able