of organs and structures that have thus followed a perfectly normal course?" we are soon able to discover that there exists between the developed organs and the external stimuli which provoked their development a mutual relation of such a kind that one is perfectly adapted to the other; in a perfectly natural state, physiological organisms are nicely adjusted to the external conditions surrounding them, in which it is natural for them to live, and which require of the organs developed just such functions as their formation and power adapt them to perform. Adaptation of the organism to its environment is, therefore, the grand purpose of the peculiarities of structure observed in different animals and plants. It is almost needless to present illustrations; they occur without number, and are quite apparent to the most superficial observer. The digestive organs are differently constructed in the herbivorous and carnivorous animals, and are thereby adapted to the different kinds of food on which the animals subsist. The gills of fish and the lungs of mammalian quadrupeds are structurally and functionally unlike; the differences adapting the breathing-apparatus of each animal to the particular kind of respiratory medium in which it is to live. And so of all other organs.
Now, if, instead of permitting the organism to remain living in its natural state, we change the surrounding natural conditions to others that are unnatural, the action of these latter will excite in the organism corresponding unnatural reactions: at first an unnatural modification of function will ensue; and in time, if the modified functions are in this manner continued, we observe a corresponding modification of structure to follow. But the modifications of structure, thus induced, are nothing else than organize diseases; they are departures from the physiological standard of health. And if we ask, "What is the conservative use and designed purpose of these unnatural deviations?" the answer is, that the modifications of structure adapt the affected organs to modified functions that they have been called upon to perform and mould the organism to new conditions that have been brought to act upon it; just as variation in the physiological construction of different animals adapts them to the various differences of surrounding media in which they are designed to live. Pathology, therefore, is really nothing else than modified physiology. Physiological development is the evolution of organs and the growth of organisms under the impression of natural external conditions; pathological development is the evolution of organs and the growth of organisms under the impression of unnatural external conditions. Adaptation of structure to function—of organisms to surrounding media—is the designed conservative purpose of both kinds of growth.
Furthermore, as no two human organisms are ever, in any particular, precisely similar, and as between organisms that have followed a strictly physiological development, and those whose development has been decidedly pathological, there are still others of intermediate
