While an instinctive desire for certain foods is peculiar to each species, it will be found that all animals in their choice of food are influenced, consciously or unconsciously, by three main considerations : the richness of the food in nutrient material, its digestibility, and its taste. In other words, the tendency of every animal is to seek out food of the most concentrated, and, to it, the most digestible and palatable kind.
The advantages of a concentrated and easily digestible food lie in the fact that the needful amount can be procured and assimilated with the minimum expenditure of energy. Of vegetable foods, seeds and roots are the most concentrated, but of all natural food animal tissue is at once the most concentrated and, by those possessed of the proper digestive organs, the most easily digested, so much so that, given the necessary facilities, even herbivorous species tend to become carnivorous, while the reverse seldom, if ever, occurs : thus in winter the reindeer eats flesh and in seasons of dearth farmers have been known to feed their cattle and even their horses on fish, but the carni-vora cannot digest vegetable food unless it has been artificially prepared.
It need scarcely be said that the palatableness of any given food varies for different species : what may be highly palatable to one species may excite disgust in another. Thus there may be species which revel in bitter and sour foods, such as aloes and sloes, but probably the tendency of all mammals, and certainly the tendency of the higher ones, is to avoid what is bitter and acrid in favour of what is sweet and possessed of delicate flavour. Within limits, sourness is tolerated by man, who sometimes manifests an actual craving for sour and even bitter things, such as lemons and various bitter herbs. No doubt the apes in their natural state subsist largely on bitter and acrid substances from which the human palate would recoil in disgust, but we may be sure that even with them the tendency is in the direction indicated.
That man tends to seek out concentrated, digestible, and delicately flavoured food in preference to food that is bulky, indigestible, and acrid, is shown by the directions in which he has modified his food-stuffs by cultivation. If we contrast wild, uncultivated vegetable foods with the corresponding cultivated varieties, we find the former differing from the latter chiefly in containing a smaller proportion of nutrient material and a larger proportion of cellulose, and in being less palatable owing to the presence in them of bitter and acrid substances; whence it is clear that the chief objects aimed at in cultivation have been, on the one hand, to diminish the proportions of the indigestible cellulose and of the disagreeable ingredients, and on the other, to increase the proportion of nutrients - starch, sugar, proteins, and fats. Other things being equal, those vegetable foods which contain the smallest proportions of cellulose (especially of the denser kind) and of bitter and acrid constituents, are the most nutritive and palatable, and therefore the most eagerly sought after. This is an important principle, explaining as it does how the evolving man gradually came to leave certain vegetable foods out of his dietary.