The dietetic changes which we have been considering have been met by corresponding changes in function and structure.
The ability to digest animal food rose and fell with the quantity consumed during the various diet epochs, and was thus greatest during the early hunting period. The Esquimaux may be regarded as being still in this stage, and doubtless their capacity to digest large quantities of animal food greatly exceeds that of the average man.
The ability to digest vegetable foods in the raw state, especially the coarser kinds, has steadily diminished from the simian stage until the present time. One can scarcely doubt that our simian ancestors had special powers of coping with cellulose, as well as with the bitter and acrid constituents met with in many vegetable foods.
Since before the discovery of cookery most of the vegetable food was subjected to prolonged mastication and in consequence intimately mixed with saliva, we may presume that in pre-cookery times salivary digestion was correspondingly active, and that the amylitic power of the salivary glands was greater than it is in the neo-man who, owing to its softness, masticates his vegetable food very little. We must suppose that during the entire cookery period there has been a waning of the function of mastication.
Seeing, moreover, that up to this period very little crude starch entered the stomach, we may assume that in pre-cookery times the digestive organs were intolerant of large quantities of starch, but that after the introduction of cookery their ability to deal with it steadily increased, becoming ultimately greatest in those communities which, like the Hindus, subsist mainly on vegetable food.
The recent large increase in the consumption of sugar is probably leading to a similar increase in the power to digest sugar.
These adaptations have taken place by natural selection, e.g. by the survival of certain types, a process which, indeed, we may see going on under our very eyes. Tens of thousands of starch-deluged and bottle-fed children are in our own country yearly weeded out, those best adapted to this mode of feeding surviving and handing down their peculiar digestive traits to their offspring.
The adaptations referred to are congenital and racial, but in considering man's digestive capacity we must not lose sight of his extraordinary personal adaptability to different kinds of food, a characteristic which has doubtless been acquired in the struggle for existence, and has given him considerable advantage over other animals in that struggle.
Of all animals the frugivorous mixed feeders exhibit the greatest adaptability to different kinds of diet. The herbivora have difficulty in digesting any food other than vegetable, while the carnivora experience a similar difficulty in digesting (in the raw state at least) any but animal food; among the mixed frugivorous feeders, on the other hand, the range of digestive capacity is considerable, being widest in that most omnivorous of mixed feeders, man, and it is largely on this account that he has been able to range far and wide over the earth, and adapt himself to the different kinds of food afforded by different regions. Thus some, like the Esquimaux, have come to be almost pure animal feeders, and others, like the high caste Hindus, to be pure vegetarians.
That man quite early in his philogenesis acquired great digestive adaptability is suggested by the fact that the anthropoid apes can be made to subsist on a diet very like that of neo-man, though doubtless man's capability in this respect has considerably increased since simian times.
This readiness of adaptation needs to be constantly borne in mind, for it shows that there is no fixed stereotyped dietary normal to man, as there is in the case of animals like the herbivora and the carnivora, but that he is capable of adapting himself to many and widely varying systems of diet, as is well attested by the very different dietetic customs prevailing in different countries.
Nor must it be forgotten that individuals differ greatly in their digestive idiosyncrasies. We meet with some, e.g. who exhibit a natural tendency to vegetarianism and a dislike of animal food, and others who, though not objecting to animal food, have some difficulty in coping with any but small quantities, especially when in the form of "meat." Such persons, arguing from themselves, are too apt to assume that man is by nature vegetarian and to decry all animal food as poisonous to him, the real truth probably being that their vegetarian leanings are atavistic in character - survivals from a far-off frugivorous past.