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amongst other changes, there is deposition of fatty matter. The phagocytes resist the influence of invading poisons better than any of the other cells of the body and sometimes are stimulated by them. The general result of such conditions is that there comes to be a struggle between the higher cells and the phagocytes in which the latter have the advantage.

The answer to the question as to whether our senescence can be ameliorated must be approached from several points of view. This course I shall now follow.

PART II

LONGEVITY IN THE ANIMAL

KINGDOM

I

THEORIES OF LONGEVITY

Relation between longevity and size-Longevity and the
period of growth-Longevity and the doubling in weight
after birth-Longevity and rate of reproduction-Probable
relation between longevity and the nature of the food

THE duration of the life of animals varies within very wide limits. Some, as for instance, the males of certain wheel animalcula (Rotifera) complete their cycle of life from birth to death in 50 or 60 hours, whilst others, like some reptiles, live more than 100 years, and quite possibly may live for two or three centuries.

Enquiry has been made for many years as to whether there are laws governing these different durations of life. Even the most casual observation of domesticated animals has shown that, as a general rule, small animals do not live so long as large ones; mice, guinea pigs, and rabbits for instance, have shorter lives than geese, ducks, and sheep, whilst these again are survived by horses, deer, and camels. Of all the mammals which have lived under the protection of man, the elephant is at once the largest, and the most long-lived.

However, it is not difficult to show that there is no absolute relation between size and longevity, since parrots, ravens, and geese live much longer than many mammals, and than some much larger birds.

As a general rule it may be said that a large animal takes more time than a small one to reach maturity, and it has been inferred from this that the length of the periods of gestation and of growth were in proportion to the longevity. Buffon1 long ago stated his opinion that the "total duration of life bore some definite relation to the length of the period of growth." Therefore, as the period of growth is, so to say, inherent in the species, longevity would have to be regarded as a very stable phenomenon. Just as any species has acquired a fixed and practically invariable size, so it would have acquired a definite longevity. Buffon, therefore, thought that the duration of life did not depend on habits or mode of life, or on the nature of food, that, in fact, nothing could change its rigid laws, except an excess of nourishment.

Taking as his standard the total period of development of the body, Buffon came to the conclusion that the duration of life is six or seven times that of the period of growth. Man, for instance, he said, who takes 14 years to grow, can live 6 or 7 times that period, that is to say, 90 or 100 years. The horse, which reaches its full size in 4 years, can live 6 or 7 times that length of time, that is to say from 25 to 30 years. The stag takes 5 or 6 years to grow, and reckoned in the same way, its longevity should be 35 to 40 years.

2

Flourens although supporting his principle, thought that Buffon had been inexact in calculating the period of growth. In his opinion a better result can be obtained by taking the limit of growth as that age at which the epiphyses of the long bones unite with the bones them

1 Histoire naturelle générale et particulière, vol. ii. Paris, 1749.

2 De la longévité humaine et de la quantité de vie sur le globe, Paris,

selves. Using such a mode of computation, Flourens laid down that an animal lived 5 times the length of its period of growth. Man, for instance, takes 20 years to grow, and he can live for 5 times that space, that is to say, 100 years; the camel takes 8 to grow, and lives 5 times as long, i.e., 40 years; the horse, 5 to grow, and lives 25 years.

reserve.

However, even if we consider only the mammalia, it is impossible to accept Flourens' law, without considerable 1 Weismann has referred to the case of the horse, which is completely adult at 4, but lives not merely 5 times that period, but 10 or even 12 times. Mice grow extremely quickly, so that they are able to reproduce at the age of 4 months. Even if we take 6 months as their period of growth, their longevity of 5 years is twice as long as it would be according to the rule of Flourens. Amongst domesticated animals, the sheep is slow in reaching maturity; it does not acquire its adult set of teeth until it is 5 years old, and cannot be regarded as adult until then. None the less, at the age of 8 or 10 years, it loses its teeth and begins to grow old, whilst by 14 it is quite senile. The longevity of the sheep, therefore, is not quite three times its period of growth.

If we turn to other vertebrates, the variations in the relation of growth and the duration of life are still greater. Parrots, for instance, the longevity of which is extremely great, grow very quickly. At the age of 2 years, they have acquired the adult plumage and are able to reproduce, whilst the smaller species are in the same condition. at the age of one. Incubation, moreover, is very short, not more than 25 days, and in some species not three weeks. None the less, parrots are birds which enjoy a 1 Ueber die Dauer des Lebens, Jena, 1882, p. 4.

2 Brehm, La vie des animaux, Mammijères, vol. ii. p. 623.

quite remarkable longevity. The incubation period of domestic geese is 30 days, and their period of growth is also short. However, they may reach a great age, cases of 80 years and of 100 years being on record. In contrast with these, ostriches, the incubation period of which is 42 to 49 days, and which take 3 years to become adult, have a relatively short life.

H. Milne-Edwards1 many years ago contended that there was no importance in the supposed law of relation between gestation and longevity. He sums up his criticism as follows: "Although the period of uterine life is longer in the horse, that animal does not live so long as a human being; and some birds, the incubation of which only lasts a few weeks, can live more than a century."

Bunge 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 91; 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.

1 Leçons sur la physiologie et l'anatomie comparée, vol. ix. 1870, P. 446.

2 Archiv f. die gesammte Physiologie, Bonn, 1903, vol. xcv. p. 606.

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