Low fertility, however, cannot itself be regarded as a cause of longevity, as there are some very fertile animals which none the less attain great ages. There are parrots which lay two or three times a year, producing six to nine eggs in each clutch. The ducks (Anatidæ) are distinguished for considerable longevity and very high fertility, each nest containing rarely less than six and sometimes as many as sixteen eggs. The common Sheldrake lays from twenty to thirty eggs. Tame ducks, in some parts of the tropics, lay an egg daily throughout the season. Wild ducks lay from seven to fourteen eggs in one nest. Ducks and geese, none the less, frequently attain considerable ages, ducks having been known to live for 29 years. Even the common fowl, which is a notoriously prolific bird, may reach an age of twenty to thirty years.
It will be said, however, that these birds are exposed to many enemies during youth. Chickens, ducklings, and goslings are ready prey for hawks, foxes and small carnivora. The longevity is possibly to be explained as an adaptation for the preservation of the species by compensating for the great destruction of the young. Weismann explains in this way the longevity of many aquatic birds and other creatures that are much preyed on. It must be noted, however, that the longevity cannot depend on the risks run by the young birds, but must have arisen independently. If this had not occurred, creatures, the young of which are destroyed in great numbers, would have ceased to exist, as many species have disappeared in geological time. The longevity of prolific animals, the young of which are destroyed in numbers, must be due to some cause which is neither fertility nor the destruction of their offspring. This cause must be sought in the physiological processes of the organism and can be attributed neither to the length of the period of growth nor to the size attained by the adults.
After having discussed various theories of the cause of the duration of life, M. Oustalet,[34] in a most interesting essay on the longevity of vertebrates, came to the conclusion that diet was the chief factor. He thinks that there is a “definite relation between diet and longevity. For the most part herbivorous animals live longer than carnivorous forms, probably because the former find their food with ease and regularity, whilst the latter alternate between semi-starvation and repletion.” There are certainly many instances which give support to the view. Elephants and parrots, for instance, are vegetarian and reach very great ages. On the other hand, there exist long-living carnivorous animals. Many observations have made it certain that owls and eagles reach great ages, and these birds live on animal food. Ravens, which live on carrion, are also notorious for the duration of their lives. There is no exact knowledge as to the ages reached by crocodiles, but although these live on flesh, it is certain that their longevity is great.
We must seek elsewhere for the real factors that control duration of life. Before stating my conclusion, I will review what is known as to the duration of life of different animals.
II.
LONGEVITY IN THE ANIMAL KINGDOM
Longevity in the lower animals—Instances of long life in sea-anemones and other invertebrates—Duration of life of insects—Duration of life of “cold-blooded” vertebrates—Duration of life of birds—Duration of life of mammals—Inequality of the duration of life in males and females—Relations between longevity and fertility of the organism
It is wonderful to what an extent the duration of life varies amongst animals, the slightest examination of the facts showing that very many factors must be involved.