110 THE VITAL FUNCTIONS. 



ducing each of the chemical changes which the 

 food must necessarily undergo during its assimi- 

 lation. We only know that all these changes 

 are slowly and gradually effected ; the materials 

 having to pass through a great number of inter- 

 mediate stages before they can attain their final 

 state of elaboration. 



Hence, whenever we can ascertain the degrees 

 of difference existing between the chemical con- 

 dition of the substance taken into the body, and 

 that of the product derived from it, we are fur- 

 nished with a kind of scale whereby we may 

 estimate the length of the process required, and 

 the amount of power necessary for its conversion 

 into that product. It is obvious, for example, 

 that the chemical changes which vegetable food 

 must be made to undergo, in order to assimilate 

 it to blood, must be considerably greater than 

 those required to convert animal food into the 

 same fluid ; because the latter is itself derived, 

 with only slight modification, immediately from 

 the blood. We accordingly find it to be an esta- 

 blished rule, that the digestive organs of animals 

 which feed on vegetable materials are remark- 

 able for their size, their length, and their com- 

 plication, when compared with those of car- 

 nivorous animals of the same class. This rule 

 applies, indeed, universally to Mammalia, Birds, 

 Reptiles, Fishes, and also to Insects ; and below 

 these we can scarcely draw the comparison, 



