At first this reasoning misleads one, but numerous objections present themselves.
The food, no doubt, has some influence upon the composition of the milk of animals of the same species, but every animal can secrete something independent of any food, just as one kind secretes musk, another castor, etc. Yet it would not be an anomaly if an excess of caseine in proportion to the other substances was a true characteristic of ruminants.
But we admit that the milk of all mammals ought to have identically the same composition if their food suffered no modifications.
What is the food of ruminants? Without doubt it is essentially vegetable, and the plants of the field constitute the element par excellence of their nurture. These plants contain a large excess of carbohydrates in proportion to the nitrogenous.
But what are these other substances? What role do they play in digestion?
They are composed in great part of fibers and cells that suffer no change in the animal economy, and which are not acted upon by the gastric juice, as proved by their occurrence in excreta. The carbon is found almost unchanged, so that the excrements of herbivoiæ, when dried, form a valuable fuel. Ruminants are compelled, in order to obtain nourishment from the plants that they eat, to extract their juices by repeated pressure (as in chewing the cud); and what do these soluble juices contain? Some saccharine substances, a little fat, but mostly albumen and vegetable caseine, that is to say, the substance which predominates in their lacteal secretions.
What, on the contrary, is the food of woman?
No doubt she gains much strength from the lean, muscular flesh that she eats, but besides this she has butter, oil, fats of all kinds, sugar, starches, and alcoholic beverages, all of which are favorable to the production of butter in the milk. Hence, aside from her physical constitution, the food of woman alone explains the relative excess of non-nitrogeneous substances.
Nitrogenous articles of food are expensive, while the other forms of nutriment are to be had in the form of potatoes, beans, and bread, products sold at a reasonable price. Yet logic demands that there shall be an excess of butter in proportion to caseine in the milk.
The discrepancies in analyses of woman's milk are easily explained by the mobile and impressible character of woman.