360 GENERAL VIEW OF THE HUMAN FUNCTIONS. 



getative life; and they are subdivided into those concerned in the development and 

 maintenance of the structure of the individual, which are termed functions of Nu- 

 trition, and those to which the Reproduction of the species is due. The great feat- 

 ure of the Nutritive operations in the Plant, is their constructive character. They 

 seem as if destined merely for the building up and extension of the fabric; and to 

 this extension there seems in some cases to be no determinate limit. But it is very 

 important to remark, that the growth of the more permanent parts of the struc- 

 ture is only attained by the continual development, decay, and renewal of parts, 

 whose existence is temporary. No fact is better established in Vegetable Phy- 

 siology, than the dependence of the formation of wood upon the action of the 

 leaves. It is in their cells that those important changes are effected in the sap, 

 by which it is changed from a crude watery fluid, containing very little solid 

 matter, to a viscid substance including a great variety of organic compounds, 

 destined for the nutrition of the various tissues. The "fall of the leaf" results 

 merely from the death and decay of its tissue ; as is evident from the fact that, 

 for some time previously, its regular functions cease, and that, instead of a fixa- 

 tion of carbon from the atmosphere, there is a liberation of carbonic acid (a result 

 of their decomposition) in large amount. 1 Now this process takes place no less 

 in " evergreens" than in " deciduous" trees; the only difference being, that the 

 leaves in the latter are all cast off and renewed together, whilst in the former 

 they are continually being shed and replaced, a few at a time. It appears as if 

 the nutritious fluid of the higher Plants can only be prepared by the agency of 

 cells whose duration is brief; for we have no instance in which the tissue con- 

 cerned in its elaboration possesses more than a very limited term of existence. 

 But by its active vital operations, it produces a fluid adapted for the nutrition 

 of parts which are of a much more solid and permanent character, and which 

 undergo little change of any kind subsequently to their complete development; 

 this want of tendency to decay being the result of the very same peculiarity of 

 constitution as that which renders them unfit to participate in the proper vital 

 phenomena of the organism ( 114). Thus the final cause or purpose of all the 

 Nutritive functions of the Plant, so far as the individual is concerned, is to pro- 

 duce an indefinite extension of the dense, woody, almost inert, and permanent 

 portions of the fabric, by the continued development, decay, and renewal of the 

 soft, active, and transitory cellular parenchyma. The Nutritive functions, how- 

 ever, also supply the materials for the continuance of the race, by the genera- 

 tion of new individuals ; since a new germ cannot be formed, any more than the 

 parent structure can be extended, without organizable materials, prepared by the 

 assimilating process, and supplied to the parts where active changes are going on. 

 369. On analyzing the operations which take place in the Animal body, we 

 find that a large number of them are of essentially the same character with the 

 foregoing, and differ only in the conditions under which they are performed; so 

 that we may, in fact, readily separate the Organic functions, which are directly 

 concerned in the development and maintenance of the fabric, from the Animal 

 functions, which render the individual conscious of external impressions, and 

 capable of executing spontaneous movements. The relative development of the 

 organs destined to these two purposes, differs considerably in the several groups 

 of Animals. The life of a Zoophyte is upon the whole much more "vegetative" 

 than " animal ;" and we perceive in it, not merely the very feeble development of 

 those powers which are peculiar to the Animal kingdom, but also that tendency 

 to indefinite extension which is characteristic of the Plant. In the Insect, we 

 have the opposite extreme; the most active powers of motion, and sensations of 

 which some (at least) are very acute, coexisting with a low development of the 

 organs of nutrition. In Man, and the higher classes generally, we have less 



1 See "Princ. of Phys., Gen. and Comp.," ^120, 494, 554, Am. Ed. 



