rather an inference from the slowness of growth than a statement of observed fact. Horses and cattle are large animals, but do not enjoy very long lives. The usual duration of life in horses is from 15 to 30 years. They begin to grow old about 10 years, and in very rare cases may reach 40 or more. A Welsh pony is said to have reached the age of sixty, but such a case is excessively rare. Two other extreme cases are that of a horse belonging to the Bishop of Metz which died at the age of 50 years, and the charger of FieldMarshal Lacy which died at 46. The duration of life of cattle is still shorter. Domestic cattle show the first sign of age, a yellow discoloration of the teeth, when five years old. In the sixteenth to eighteenth year the teeth fall out, or break, and the cow ceases to give milk, whilst the bull has lost reproductive power. According to Brehm, cattle live for 25 to 30 years or more. Although the duration of life is short, cattle are not prolific. The gestation period of a cow approaches that of the human race (242-287 days), and there is only one birth a year. The total period of reproductivity lasts only a few years. The sheep, another domesticated Ruminant, has a life even shorter. According to Grindon, sheep do not live longer than 12 years as a rule, but may reach 14 years, which in their case would be extreme age, as they generally lose their teeth at from 8 to 10 years. Some Ruminants, such as camels and deer, apparently live longer than sheep or cattle, but I do not know exact facts about them. The short life of domesticated carnivorous animals is well known. Dogs seldom live more than 16 or 18 years, and even before that, at an age of from 10 to 12, they usually show plain signs of senility. Jonatt has mentioned as an extreme rarity a dog of 22 years of age, and Sir E. Ray Lankester (Comparative Longevity, p. 60) cites another instance, in this case the age being 34 years. The oldest dog that I have been able to procure died at the age of 22. It is generally believed that cats do not live so long as dogs. The average age which they may attain is usually thought to be 10 or 12 years, but certainly a cat of that age has not the decrepid appearance of an old dog. Thanks to the kindness of M. Barrier, the Director of the Ecole d'Alfort, I have had in my possession a cat 23 years old. It appeared to be quite vigorous, and died from cancer in the liver. Most rodents, particularly the domesticated kinds, are extremely prolific and very short lived. It is extremely rare for a rabbit to reach the age of 10 years, whilst 7 years is the utmost limit for a guinea-pig. Mice, so far as I can ascertain, do not live more than 5 or 6 years. It is plain from the facts that I have brought together, that mammals, whether they are large or small, as a rule, have shorter lives than birds. It is probable, therefore, that there is something in the structure of mammals which has brought about a shortening in the duration of their lives. Whilst most of the lower vertebrates, and all birds, reproduce by laying eggs, the vast majority of mammals are viviparous. As the tax on the parent organism is greater when the young are produced alive than when eggs are laid, it might be thought that in this difference lay the cause of the shorter life of mammals. It is well known that an animal may be made feeble by too great fecundity, and it is conceivable that the kind of parasitic life of the embryos within the body of the mother may weaken her system. There are many facts, however, which make it impossible to accept such a view. The longevity of mammals. is nearly equal in the two sexes, although the tax on the organism caused by reproduction is much greater in the case of females than in males. Longevity, however, cannot be regarded as a character stable in each species and necessarily identical in the two sexes. The animal kingdom presents many cases of disparity in this respect, the difference in longevity in the two sexes being specially striking in species of insects. Generally, the females live longer than the males, as, for instance, amongst the Strepsiptera, where the females have 64 times the duration of life of the males. On the other hand, amongst butterflies, there are cases (e.g., Aglia tau) where the males live longer than the females. In the human race, there is a difference in the longevity of the sexes, the females having the advantage. As in most cases of disparity in the duration of life the female lives longer than the male, it is plain that the difference cannot be assigned to the drain on the organism caused by reproduction, which, of course, is much greater in females. Moreover, a closer scrutiny of the facts shows that although mammals do not live so long as birds, the reproductive drain is greater in the case of birds. It is well known that the productivity of an animal is not necessarily identical with its fecundity. Fish or frogs which lay thousands of eggs at a time (a pike, for example, produces 130,000) are obviously more prolific than, for instance, a sparrow which lays only 18 eggs in a year, or than a rabbit, which in the same time gives birth to from 25 to 50. However, to produce this much smaller quantity of eggs or of young, the sparrow and the rabbit (I have chosen the most prolific bird and mammal) expend a much larger quantity of material than the frog or the fish. The sparrow and the rabbit employ in producing their progeny a bulk of material greater than the weight of their body, whilst the enormous quantity of eggs laid by the frog does not weigh more than one-seventh part of the body of the frog. It may be laid down, as a general rule, that although fecundity, that is to say the number of eggs or of young which are produced, diminishes as the organism becomes more complex, the productivity on the other hand increases, expressed in percentage of weight. The productivity, which is not more than 18 per cent. in batrachia, reaches 50 per cent. in reptiles, 74 per cent. in mammals, and 82 per cent. in birds. It is plain that if reproduction shortens the life of mammals by weakening the organism, it must be the productivity, not the fecundity, which is the important factor. I have just shown that productivity is greater in birds than in mammals, and in consequence it cannot be on account of any greater burden of reproduction that mammals have a shorter life than birds. The shortness of mammalian life, again, cannot be attributed to the fact that mammals give birth to young, whilst the long-lived reptiles and birds produce eggs, because the longevity of the males, which produce neither young nor eggs, is none the less practically equal to that of the females of the same species. The reason of the short life of mammals must be sought for elsewhere. III THE DIGESTIVE SYSTEM AND SENILITY Relations between longevity and the structure of the digestive We have seen that the duration of life in mammals is relatively shorter than that in birds, and in the so-called cold-blooded " vertebrates. No indication as to the cause of this difference can be found in the structure of the organs of circulation, respiration, or urinary secretion, or in the nervous or sexual apparatus. The key to the problem is to be found in the organs of digestion. In reviewing the anatomical structure of the digestive apparatus in the vertebrate series, one soon comes to the striking fact that mammals are the only group in which the large intestine is much developed. In fish, the large intestine is the least important part of the digestive tube, being little wider in calibre than the small intestine. Amongst batrachia, where it is a relatively wide sack, it has begun to assume some importance. In several reptiles it is still larger, and may be provided with a lateral out-growth, which is to be regarded as a cæcum. In birds, the large intestine still remains relatively badly developed; it is |