SOURCES OF DEMAND FOR FOOD. 237 



and the Frog in the condition of a Fish. But it is now known that 

 changes of form, as complete as these, occur in a large proportion of 

 the lower tribes of Animals ; so that the absence of them is the excep- 

 tion. The fact seems to be, that the supply of nutriment laid up within 

 the egg, among the lower classes, is by no means sufficient to carry on 

 the embryo to the form it is subsequently to attain; and its development 

 is so arranged, that it may come into the world in a condition which 

 adapts it to obtain its own nutriment, and thus to acquire for itself the 

 materials for its further development. Thus the Insect, in its larva or 

 Caterpillar state, is essentially a foetus in regard to its grade of develop- 

 ment ; but it is a foetus capable of acquiring its own food. In this con- 

 dition it attains its full growth as regards size, though its form remains 

 the same ; but it then, in passing into the Chrysalis state, reassumes (as 

 it were) the condition of an embryo within the egg, the development 

 of various new parts takes place, at the expense of the nutriment stored 

 up in its tissues, and it comes forth as the perfect Insect. In many 

 of the lower tribes, the animal quits the egg at a still earlier period in 

 comparison ; thus it has been lately shown by M. Milne Edwards, that 

 some of the long marine worms consist only of a single segment, form- 

 ing a kind of head, when they leave the egg ; and that the other seg- 

 ments, to the number (it may be) of several hundred, are gradually 

 developed from this, by a process that resembles the budding of Plants. 



408. Up to the period, then, when the full dimensions of the body 

 have been attained, and the complete evolution of all its organs has taken 

 place, a due supply of food is necessary for these purposes. In the 

 Plant, nearly the whole of the alimentary materials taken into the 

 system, are thus appropriated ; the extension of its structure going on 

 almost indefinitely, and the waste occasioned by decay being compara- 

 tively small. Thus the carbon, which is given out by the respiratory 

 process in the form of carbonic acid, bears but a small proportion to 

 that which is introduced by the decomposition of that same gas, under 

 the influence of light ( 81). And the fall of the leaves, which takes 

 place once a year or more frequently, and which gives back a large 

 quantity of the matter that has undergone the organizing process, does 

 not occur, until by their means a considerable addition has been made 

 to the solid and permanent substance of the tree. 



409. This is not the case, however, with the animal. Its period of 

 increase is limited. The full size of the body is usually attained, and 

 all the organs acquire their complete evolution, at a comparatively early 

 period. The continued supply of food is not the ; n requisite for the ex- 

 tension of the structure, but simply for its maintenance ; and the source 

 of the demand lies in the constant waste, to which, during its period of 

 activity, it is subjected. We have seen that every action of the 

 Nervous and Muscular systems involves the death and decay of a cer- 

 tain amount of the living tissue, as is indicated by the appearance of 

 the products of that decay in the Excretions ; and a large part of the 

 demand for food will be consequently occasioned by the necessity for 

 making good the loss thus sustained. Hence we find that the demand 

 for food bears a close relation to the activity of the animal functions ; 

 so that a diet, which would be superfluous and injurious to an individual 



