172 THE POPULAR SCIENCE MONTHLY. 



pursue such a typical course, the observed structural changes are ex- 

 ceedingly slow, and are attended with little or no physical sujfering. 



Let us speak, fii'st, of physiological evolution. And, to begin, let 

 us ask, " How shall we know when physiological development is fol- 

 lowing its natural course, and when it is not ? " There are two strik- 

 ing 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 saltern. Ab- 

 rupt 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 consid- 

 erable 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 devel- 

 opment 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, con- 

 sists in ioinorance or unconsciousness of the existence of the oro;ans 

 performing it, so is it an invariable quality of every perfectly natural 

 organic change tliat 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 per- 

 fectly 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 exist- 

 ing differences being so inappreciable as to evade ordinary methods 

 of observation. This uniformity of type, however, will only be ob- 

 served in wild animals that have led a strictly natural life, untram- 

 meled 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 in- 

 stances are the nutritive changes of waste and repair, growth and de- 

 velopment, that not only are the size and shape uniformly the same, 



