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 system—The Cæca in birds—The large intestine of mammals—Function of the large intestine—The intestinal microbes and their agency in producing auto-intoxication and auto-infection in the organism—Passage of microbes through the intestinal wall