148 THE INDUCTIONS OF BIOLOGY. 



food may stop the formation of a mammal's skeleton: thus 

 dwarfing, if not eventually destroying, the mammal; and 

 this no matter what quantities of other needful colloids and 

 crystalloids are furnished. 



Again, the truth that, other things equal, growth varies 

 according to the supply of nutriment, has to be qualified by 

 the condition that the supply shall not exceed the ability to 

 appropriate it. In the vegetal kingdom, the assimilating 

 surface being external and admitting of rapid expansion by 

 the formation of new roots, shoots, and leaves, the effect of 

 this limitation is not conspicuous. By artificially supplying 

 plants with those materials which they have usually the most 

 difficulty in obtaining, we can greatly facilitate their growth; 

 and so can produce striking differences of size in the same 

 species. Even here, however, the effect is confined within 

 the limits of the ability to appropriate; since in the absence 

 of that solar light and heat by the help of which the chief 

 appropriation is carried on, the additional materials for 

 growth are useless. In the animal kingdom this 



restriction is rigorous. The absorbent surface being, in the 

 great majority of cases, internal; having a comparatively 

 small area, which cannot be greatly enlarged without re- 

 construction of the whole body; and being in connexion 

 with a vascular system which also must be re-constructed 

 before any considerable increase of nutriment can be made 

 available ; it is clear that beyond a certain point, very soon 

 reached, increase of nutriment will not cause increase of 

 growth. On the contrary, if the quantity of food taken in is 

 greatly beyond the digestive and absorbent power, the excess, 

 becoming an obstacle to the regular working of the organism, 

 may retard growth rather than advance it. 



While then it is certain, a priori, that there cannot be 

 growth in the absence of such substances as those of which 

 an organism consists ; and while it is equally certain that the 

 amount of growth must primarily be governed by the supply 

 of these substances; it is not less certain that extra supply 



