at least during the first few days of life, but the liability continues, though in a gradually-decreasing ratio, until physiological development is complete. Breeders of stock know very well the difficulty of rearing the young; and even in wild animals that live in a perfectly natural state, untrammeled by domestication, a good proportion of new-born individuals perish in early life. Gardeners and agriculturists drop more seeds in a place (of corn "in a hill," for instance) than they intend shall remain as plants, knowing that in the earlier stages of growth many of the "seedlings" will die; and, if more shall remain than is advisable, they are afterward "thinned" by hand. Surely every mother of a family knows the difficulty of rearing children, and our "bills of mortality" sufficiently attest the immense fatality attending physiological development in the human family.
Now, while in each of the instances I have cited the causes and mode of death are similar to or at least analogous with each other, it is only in the case of our own species that, generally speaking, we say death has been caused by disease. In the young chick, the wild animal, and the seedling plant, we are content to say they die because they are young, or "tender." The truth is, that death has been due, in each case, to an arrest of, or interference with, the quite normal process of physiological development, and to put this conception to a further test we may begin still a little earlier in life by studying embryonic development. In the case of oviparous animals, for example, we know many of the eggs never come to perfection; the young embryos they contain die during incubation. And while the pathologist, if he were to delve with his microscope into the secret physiology and pathology of the growing embryo, might find different membranes and organs fatally affected (congested or inflamed) according to the different stages of development at which the mortal disturbance took place, it would seem very odd, in case it should happen to be the rudimentary lungs of the growing embryo that were found specially congested, if he should say the egg had been "attacked" with pneumonia (inflammation of the lungs); or, if he should find the heart or intestines congested, how queer it would sound to say the egg had died from an attack of carditis (inflammation of the heart), or enteritis (inflammation of the intestines)! yet these so-called diseases are just as much causes of death in the egg as in the child after it is born.
The relevancy of the facts mentioned to the question at issue—the bearing of the argument—is this: organisms undergoing physiological evolution, and those in which pathological evolution is going on, are alike liable to be fatally affected by certain disturbing causes that interfere with the typical progress of development in each; hence the great mortality incident to childhood and early life is strictly analogous with the mortality attending organic diseases in the adult.
This analogy may now be further sustained by considering what these disturbing causes really are; and here I may premise they will
