Page:Popular Science Monthly Volume 7.djvu/184

172 pursue such a typical course, the observed structural changes are exceedingly slow, and are attended with little or no physical suffering.

Let us speak, first, of physiological evolution. And, to begin, let us ask, "How shall we know when physiological development is following its natural course, and when it is not?" There are two striking characteristics which can always here be taken as guides, viz., slowness of organic change, and latency. The physiological growth of tissue is always gradual in its progress, both as regards change of shape, bulk, and variation in physical properties. These go on with an insidious progression that is, for the most part, quite imperceptible to ordinary methods of observation. Natura non facet saltem. Abrupt changes of shape, size, etc., incident to function, are, of course, quite common, but these are only temporary, and quite distinct from the more stable organic changes constituting growth. Indeed, it may be taken as an invariable rule that structural changes of any considerable extent, that occur abruptly and remain permanent, are always unnatural, and are to be attributed to the action, direct or indirect, of some disturbing cause.

Secondly, we notice that the natural course of physiological development is characterized by painlessness—unconsciousness on the part of the individual that any tissue-changes at all are taking place. As the height of perfection in function, as in digestion, for example, consists in ignorance or unconsciousness of the existence of the organs performing it, so is it an invariable quality of every perfectly natural organic change that it should take place without the knowledge of the individual—I mean without any knowledge derived from unpleasant sensations. No argument is needed to support this statement; we will only add that, when physiological organic changes are accompanied with pain, it is always attributable to some injurious influence leaving caused the changes that are taking place to deviate from their natural and typical course.

A third characteristic of undisturbed physiological evolution is this: the developed organs, when their evolution is complete, and indeed during their development, present a typical uniformity of structure; that is to say, organisms whose development has been perfectly natural, provided they are of the same age, sex, species, etc., are alike; and the several organs of different individuals present a uniform standard of size, shape, and functional power, any slight existing differences being so inappreciable as to evade ordinary methods of observation. This uniformity of type, however, will only be observed in wild animals that have led a strictly natural life, untrammeled by domestication, such as ocean and river fish that have not been removed from their natural waters, wild birds, reptiles, insects, and the untamed mammalian animals. So exactly similar in such instances are the nutritive changes of waste and repair, growth and development, that not only are the size and shape uniformly the same,