318 DIFFERENTIATION OF STBUCTUEE. 



animals ; for in some it is checked so early, that scarcely any 

 distinction either of organs or of tissues ever shows itself; 

 whilst in others it continues during a large proportion of the 

 earlier period of life. It has no relation to growth, or simple 

 increase of size ; for this may take place by the multiplication 

 of similar parts, giving rise to that "vegetative repetition" 

 which is so characteristic of the lower tribes of Animals, and 

 which gives to many of them so strong a resemblance in 

 general aspect to Plants ; whilst, on the other hand, the de- 

 velopmental process by which higher forms of structure are 

 evolved, sometimes takes place without any increase at all. 

 It is in its degree of such differentiation, that what is called the 

 lower or the higher organization of any living being essentially 

 consists ; for whilst in the simplest forms of Animal struc- 

 ture every part is similar to every other, so that all the 

 functions of life are performed in common by each, we find 

 in Man (whose body may be regarded as presenting the 

 highest type or example of this differentiating process) that 

 no two parts are precisely similar, except those on the 

 opposite sides of the body. This fact is occasionally mani- 

 fested in a very singular manner, in the symmetry of disease ; 

 certain morbid poisons (as those of gout, and of several affec- 

 tions of the skin), which have a tendency to single out par- 

 ticular spots of the tissues whose nutrition they disturb, 

 exhibiting their action in those parts of the two lateral halves 

 of the body which precisely correspond with each other. 



381. Now in the lowest grades of Animal structure, we find 

 that the several tissues of the body can themselves appropriate 

 from the products of digestion the nutrient materials they 

 respectively require ; so that, for their growth and mainte- 

 nance, it is sufficient that these products should be carried 

 into their neighbourhood by extensions of the digestive cavity 

 ( 296). But in all the -more highly-organized animals, it 

 appears requisite that the nutrient material should pass 

 through an intermediate stage of preparation, which is termed 

 assimilation (or making-like); and this is effected by their 

 introduction into the current of the circulation, and their 

 mixture with the pre-existing blood, which, in virtue of its 

 own vital powers, exerts upon them a converting action, which 

 prepares them for being appropriated by the solid tissues. 



382. When once the several forms of tissue have been 



