318 DIFFERENTIATION OF STRUCTURE. 
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 eavi i 
($ 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 | it ‘ 2 
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 beets 1 
