FORMATIVE POWER OF INDIVIDUAL PARTS. 547 



from the original germ, but undergoing a gradual diminution with the advance 

 of life, until the power of maintenance is no longer adequate to antagonize the 

 forces that tend to the disintegration of the system (CHAP. HI., SECT. 3). It 

 has been also shown that, notwithstanding the diversities in the structure and 

 composition of the several tissues, the Blood supplies the materials which each 

 requires ; every tissue possessing (so to speak) an elective affinity for some par- 

 ticular constituents of that fluid, in virtue of which it abstracts them from it 

 and appropriates them to its own uses. But it has been shown, on the other 

 hand, that the " formative capacity" does not exist in the tissues alone, but is 

 shared by the Blood, which must itself be regarded as deriving it from the 

 original germ ; for there are certain simple kinds of tissue, which seem to take 

 their origin directly in its plastic components ( 26-29). Of others which 

 cannot be said thus to originate in the blood, the development is entirely deter- 

 mined by the quantity of their special pabula which it may contain ( 120). 

 And even of those tissues which must be considered as most independent and 

 self-sustaining, the development is not only checked by the want of a due supply 

 of their appropriate materials, but it is modified in a very remarkable degree by 

 the presence of abnormal substances in the blood, which single out particular 

 parts, and effect determinate alterations in their nutrition, in such a constant 

 manner as to show the existence of a peculiar "elective affinity" between them 

 ( 201). In so far, then, as the process of Nutrition is dependent upon the due 

 supply and normal state of the Blood, its conditions have been already suffi- 

 ciently discussed ; and we have now only to consider it in its relations to the 

 Tissues. 



586. The demand for Nutrition primarily arises from the tendency of the or- 

 ganism to simple increase or growth. Of this we have the most characteristic 

 illustration in the multiplication of the first embryonic cell, by the simple pro- 

 cess of "duplicative subdivision," ( 104;) whereby a multitude of cells is pro- 

 duced, every one of which is similar in all essential particulars to the original. 

 But after the different parts of this homogeneous embryonic mass have taken 

 upon themselves their respective modes of development, so as to generate a 

 diversity of tissues and organs, each one of these continues to increase after its 

 own plan ; and thus the child becomes the adult, with comparatively little change 

 but that of growth. An excess of growth, taking place conformably to the 

 normal plan of the tissue or organ, constitutes Hypertrophy ; whilst a diminu- 

 tion, without degeneration or alteration of structure, is that which is properly 

 distinguished as Atrophy. But Growth is not confined to the period of increase 

 of the body generally ; for it may manifest itself in particular organs or tissues, 

 as a normal operation, at any subsequent part of life. Thus when there is an 

 extraordinary demand for the functional activity of a particular set of Muscles, 

 it is supplied by an increase in the amount of their contractile tissue ( 312) ; . 

 or if one of the Kidneys be disabled from performing its office, the other may be 

 rendered capable of fulfilling it, by an agumented production of its own secretory 

 tissue ; or if there be an excess of fatty matters in the blood, they may be elimi- 

 nated by an augmentation of the Adipose tissue throughout the body. And 

 further, even where there is no such manifestation of increase, there is really a 

 continual growth in all the tissues actively concerned in the vital operations, and 

 this even to the very end of life ; although it may be so far counterbalanced, or 

 even surpassed, by changes of an opposite kind, that, instead of augmentation in 

 bulk, there is absolute diminution. 



587. The evolution of the complete organism from its'germ, however, does not 

 consist in mere growth ; for by such a process nothing would be produced but 

 an enormous aggregation of simple cells, possessing little or no mutual depend- 

 ence, like those which constitute the shapeless masses of the lowest Algae. In 

 addition to increase, there must be development ', that is, a passage to a higher 



