Bunge[33] has recently taken up the study of the relations between the duration of growth and longevity, and has suggested a new means of investigation. He has observed that the period in which the new-born mammal doubles its weight is a good index of the rapidity of its growth. He has shown that whilst a human child requires 180 days to reach double its weight at birth, the horse, the longevity of which is very much less, doubles its weight in 60 days; a calf takes only 47 days for this; a kid 15 days; a pig 14 days; a cat 9-1/2; and a dog only 9 days. Although these facts are very interesting, the exceptions are too great to make it possible to base a law of longevity upon them. The period of weight-doubling in the horse is nearly 7 times longer than that in the dog, and yet the longevity of the horse is not more than 3 times that of the dog. The goat, which takes much longer than the dog to double its weight, has a shorter total life. I observed myself that new-born mice quadruple their weight in the first 24 hours. The doubling of weight in their case requires a time 36 times less long than that of the cat, and yet the cat lives only 5 times as long as the mouse. It is fair to say, however, that Bunge himself does not draw a definite conclusion from these figures and has published them only to stimulate interest in the subject. He is against the view of Flourens, and points out that although the multiple 5 is valid for man, it is not so in the case of the horse which finishes its growth in 4 years and yet reaches the age of 40 much less often than human beings attain that of 100 years. Although it is impossible to admit the existence of exact relations between size and the period of growth on the one side, and longevity on the other, in the mode which Buffon and Flourens have followed, it is none the less true that there is something intrinsic in each kind of animal which sets a definite limit to the length of years it can attain. The purely physiological conditions which determine this limit leave room for a considerable amount of variation in longevity. Duration of life therefore, is a character which can be influenced by the environment. Weismann in his well-known essay on the duration of life, has laid stress on this side of the problem. Longevity, according to him, although in the last resort depending on the physiological properties of the cells of which the organism is composed, can be adapted to the conditions of existence and influenced by natural selection, like other characters useful for the existence of the species.