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 short and straight. In most birds, at the point where the large intestine passes into the small intestine, there is a pair of cæca, more or less developed. These cæca are absent in climbing birds, such as the wood-pecker, the oriole, and many others. They are reduced to a pair of tiny out-growths in the eagles, sparrow-hawks, and other diurnal birds of prey, and in pigeons, and perching birds. These organs are larger in the nocturnal birds of prey, in gallinaceous birds, and in ducks, etc.1 In the large running birds, such as ostriches, rheas, and tinamous, the cæca are relatively largest. Thus, for instance, in a rhea (Rhea americana) which I dissected, the cæca were nearly two-thirds as long as the small intestine. The latter was 1'65 m. in length, whereas one of the cæca was 'o m., and the other o'95 m. The weight of the two cæca with their contents was more than 10 per cent. of the total weight of the bird. Notwithstanding the exceptions, which are relatively rare, the large intestine is badly developed in the case of birds. On the other hand, it reaches its largest size amongst mammals. In these animals, "only the posterior portion of the latter, or rectum, which passes into the pelvic cavity, corresponds to the large intestine of lower Vertebrates; the remaining, and far larger part, must be looked upon as a neomorph, and is called the colon." 2 3 Gegenbaur, another well-known authority on compara 1 J. Maumus, "Les cæcums des oiseaux," Annales des sciences naturelles, 902. See also P. Chalmers Mitchell, "On the Intestinal Tract of Birds," Trans. Linnæan Soc. of London, vol. viii. part 7, 1901. 2 Weidersheim, Elements of the Comparative Anatomy of Vertebrates, translated by W. Newton Parker, p. 236, 1886. 3 Elements of Comparative Anatomy, English translation by F. Jeffrey Bell, B.A., London, 1878, p. 562. tive anatomy, writes as follows on this subject :-"The hind-gut is longest in the Mammalia, where it forms the large intestine, and is distinguished as such, from the mid-gut, or small intestine. Owing to its greater length, it is arranged in coils, so that the terminal portion only has the straight course taken by the hind-gut of other Vertebrata." The two series of facts are not to be disputed. On the one hand mammals are shorter lived than birds and lower vertebrates, on the other hand the large intestine is much longer in them than in any other vertebrates. Is there here any link of causality, binding the two characters, or is it a mere coincidence? To answer the question we must turn to the function of the large intestine in vertebrates. In the lower members of the group (fish, batrachia, reptiles, birds, etc.), the large intestine is not more than a mere reservoir for the waste matter in the food. It takes no share in digestion, as that is the function of the stomach and the small intestine. Only the cæcum can be thought to have some digestive property. In reptiles, the lowest vertebrates in which the cæcum is present, it is so little differentiated from the large intestine itself, that it is difficult to assign to it any specialised function. In very many birds, however, the cæca are well separated from the main digestive tube. The food material passes into them in considerable quantities, and is retained there sufficiently long for some digestive process to take place. M. Maumus has found, in the cæca of birds, secretions which can dissolve albumen and invert sugar cane, but he has been unable to make out that the cæcal juice has any action upon fatty matter. Such digestive power, however, is slight, and when M. Maumus removed the cæca in fowls and ducks, no evil consequences followed. As in many birds the cæca are rudimentary and in others absent, it may be inferred that these organs are useless, and are in process of degeneration in the class. The cæca can be regarded as playing an important part in the organism only in the case of large running birds, where they are very highly developed, but we have not precise information as to their digestive function. The variations in the structure in the large intestine are greater in mammals than in birds. In some mammals, the large intestine is a simple prolongation of the small intestine, similar in calibre and in structure. In these conditions it may fulfil a definite digestive function. Th. Eimer has determined that in insectivorous bats the large intestine digests insects like the small intestine. Such cases, however, are rare. In most mammals the large intestine is sharply separated from the small intestine by a valve, and opens directly into the cæcum which may be very large. In the horse, the cæcum is an enormous bag, cylindrical and tapering, generally well filled, and holding on an average 35 litres. It is equally large in many other herbivorous animals, such as the tapir, the elephant, and most rodents. In such cases, the food remains for a considerable time in the organ and without doubt undergoes some digestive changes. In many other mammals, particularly carnivorous forms, the cæcum may be quite absent, whilst in some, as for instance, the cat and dog, it is very small; in the latter cases its digestive function must be non-existent or insignificant.2 As for the large intestine itself, apart from the special 1 Virchow's Archiv, 1869, vol. xlviii. p. 151. 2 P. Chalmers Mitchell, "On the Intestinal Tract of Mammals," Trans. Zool. Soc. of London, vol. xvii. part 5, 1905. |