220 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



organs, which thus become shaped by use, in the higher animals, 

 under the controlling agency of the sympathetic nervous system. 

 But inasmuch as living tissue is in no two organisms precisely alike 

 in its qualities, it follows that this "use" will manifest itself in 

 different ways, even when the conditions under which they exist 

 appear to be the same. 



Plants and animals differ from inanimate things in that they 

 grow from within, by a process of "intussusception", and not from 

 accretions from without, as in a crystal. Now this growth, in living 

 tissue, is made possible by taking up substances external to the body, 

 and in some mysterious way converting them into living tissue. The 

 plants can extract this life-forming material from the mineral world, 

 but animals can only obtain it from other organic bodies, living or 

 dead. It goes, we say, to repair "waste." Let me cite an instance of 

 this kind. One goes for a long walk, returning tired and hungry. 

 After a good square meal, that stomach-hunger is appeased, and is 

 followed by a sense of satisfaction. But there was also a muscle and 

 nerve hunger of which the pedestrian, unless he were a physiologist, 

 would not be conscious. Now that "good square meal", in a very 

 short time, was reduced in the alimentary canal to a state resembling 

 salad-cream, enabling it to be taken up by the lacteals, and passed, 

 drop by drop, into the left subclavian vein at its junction with the 

 internal jugular, and thence to be carried by the blood-stream to the 

 tissues most in need of it — those wasted by the exertions of walking. 

 They would take most of that "repair substance", the rest of the 

 tissues of the body would absorb according to their need, and what 

 was still to spare would be used up to form new tissues, or, as we say, 

 to promote growth. And so it comes about that the tissues which are 

 used most will take up most of what we call the "food material", 

 growth being determined by the measure of use. 



But in this process of growth, in course of time, the form of the 

 whole body may become materially changed. And how this comes 

 about I propose to show by examples chosen from many different 

 types of animals. But I would first draw attention to another aspect 

 of these tissues, for it is one of no small importance. 



They display a curious power of "selecting" substances from their 

 food, which give them qualities confined to particular species of ani- 

 mals, often closely related. Hence the different qualities and savors 

 of beef and mutton. 



Everyone must have noticed, in carving a grouse, that the upper 

 layer of the two great breast muscles is conspicuously dark-colored, 

 while the lower is white. In the pheasant both are white. We can, 

 at present, assign no reason for this curious fact, which was mani- 

 fested, not on the dissecting table, or by any process of analysis save 



