ASSIMILATION. 



223 



mechanically ; as little can it be accounted for chemi- 

 cally ; nor do \ve succeed better when we attempt to 

 explain it by a combination of the two. There are 

 mechanical changes no doubt, and there are also che- 

 mical changes, but there are others which are neither 

 mechanical nor chemical, and in these we neither 

 know the cause nor the process. It is true that they 

 are invariable concomitants of life, and that we never 

 can find them but in the living subject. Therefore 

 we may say that they are actions of the principle of 

 life, or the living principle. But either of these 

 expressions is precisely of that nature of which we, 

 on a former occasion, mentioned that the word 

 " instinct" is an example ; they enable us to fill up by 

 sound in our words those gaps which the failure of our 

 knowledge leaves in the sense of what we say, and 

 they do nothing more. These necessary ignorances 

 do not, however, lessen the value of that which we 

 really know, only it is important for us to know where 

 they lie, because that prevents us from wasting our 

 time and labour where there is no knowledge to be 

 found ; and had we always been aware of and avoided 

 them, all of us might, with the same exertion, have 

 been very much wiser than we are. 



Taking these precautions with us, to prevent us 

 from wandering into idle speculations, the subject of 

 assimilation becomes one of the most interesting that 

 can occupy our thoughts. It is indispensable to the 

 growth of' all organic beings, whether animal or vege- 

 table ; and it is equally essential to their being in a 

 healthy state. It forms, indeed, one of the most 

 unequivocal criteria of organic life, for there is nothing 

 analogous in the inorganic or the dead world. The 

 growth of stones and the formation of crystals, have 

 sometimes been adduced as instances analogous to 

 vegetable, and even to animal growth ; but there is 

 no assimilation in these; the substance though changed 

 in form is not changed in nature, either by accretion 

 into a stone, or by arrangement into a crystal. 



As we are in a great measure ignorant of the food 

 of vegetables and their mode of feeding, any thing 

 that can be said about assimilation in them, will be 

 much better left to VEGETABLE PHYSIOLOGY, or to 

 some of the articles subordinate to that. This is 

 further desirable, because perennial vegetables, which 

 continue to increase by growth for more than one 

 season, grow by a sort of accretion, or the mere addi- 

 tion of new parts, while the portion once grown never 

 increases or undergoes any change except what may 

 be impressed upon it by causes external to itself. No 

 doubt, in the animal kingdom, pearl shells grow some- 

 thing in the same manner, as may be seen in the 

 successive additions to the shell of an oyster or a peri- 

 winkle, but crusts grow more upon the animal princi- 

 ples ; and probably the porcelain shells grow some- 

 thing in the same manner, though that is a subject 

 upon which we have very little information. 



We shall render the few remarks which we have 

 to make more clear, and therefore give them a chance 

 of being more generally acceptable and useful, if we 

 narrow the subject still further, and consider assimi- 

 lation as it takes place in vertebrated animals only. 



It is probable, nay certain, that the process of assi- 

 milation is coeval with the very first development of 

 the animal germ, nay, with the very commencement 

 of its existence, as an organic being. This part of 

 the subject is exceedingly obscure, and as is the case 

 with all obscure subjects which are at the same time 

 interesting, there are many theories and opinions 



respecting it. The most rational of these opinions, 

 and the one which accords best with the analogy of 

 nature, and carries us through the other parts with 

 the greatest security from error and absurdity, is 

 that which, we believe, was first advanced by the 

 great Harvey. This is what is usually termed the 

 theory of Epigenesis, or that which dates the very 

 first rudimental existence of the animals from "the act 

 of organisation," and denies the existence of all pre- 

 existent germs, except as mere materials to be ren- 

 dered available by that act. In no other way is it 

 possible to account for the production of mules, or 

 even for that more universal sort of hybridism, by 

 which the progeny generally partakes in some degree 

 of the qualities of both parents. But though assimi- 

 lation goes on in those early stages, and goes on in 

 a more extraordinary manner than at any future 

 period, inasmuch as it makes the whole organic struc- 

 ture of the animal, the process there is so exceed- 

 ingly nice, and so little is known about it, that it can- 

 not possibly be rendered useful or even intelligible 

 in popular description. We shall, therefore, still 

 further narrow our observations, by confining them 

 to the process of assimilation in the animal, after it is 

 capable of subsisting by itself upon the ordinary food 

 of its species. 



We shall suppose that the whole of the matter 

 which is assimilated is taken by the mouth as food ; 

 for though there are other circumstances, such as the 

 state of the atmosphere, and the kind and degree 

 of action to which the body of the animal is sub- 

 jected, which affect assimilation, yet they form no 

 specific part of it, and can be regarded as affecting it 

 only as they affect the other functions, that is, as 

 external circumstances. 



The first process which the food undergoes may 

 be said to be purely mechanical, and might be done 

 by other means. This process is mastication, or 

 chewing, on the part of many of the mammalia ; 

 trituration or grinding in gizzard birds ; and macera- 

 tion, or softening, and solution in a fluid in some 

 other animals ; but in whatever way it is performed, 

 it is a merely preparatory process, not at all con- 

 nected with assimilation ; and when there is a liquid 

 in the case, it is to be considered only as a mechani- 

 cal solvent, that is, as softening or dissolving the food 

 with which it mixes, much in the same way that 

 water softens and dissolves glue. 



This preparatory operation, and indeed the whole 

 process of assimilation, depends a good deal upon 

 the nature of the food. When that is entirely animal, 

 the whole of the processes, and also the apparatus 

 by which they are performed, are much more simple 

 than when it is wholly vegetable ; and even in the 

 case of vegetable food, that which is wholly pulpy 

 and farinaceous, requires a simpler apparatus and pro- 

 cesses than that which is mixed with fibrous matter. 

 The adaptations of all the parts of this apparatus 

 (which, taken together, are called the digestive sys- 

 tem, or nourishing system, of the animal) to the vari- 

 ous kinds of food upon which animals live, exhibit 

 some of the most beautiful instances of means and 

 end that are to be met with in the whole economy of 

 nature ; and they are also of great value in that natural 

 classification of animals, by meais of which the history 

 of one is made to throw light upon the history of a 

 number, and one part of the history of one is made 

 to throw light upon the other parts. The general 

 law is, that the nearer the food in the state in which 



