120 THE INDUCTIONS OF BIOLOGY. 



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 

 difiiculty 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 of 

 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 must also 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 nutriment 

 taken in, is greatly beyond the 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 prioriy 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 

 will not produce extra growth, beyond a point very soon 

 reached. Deduction shows to be necessary, as induction 

 makes familiar, the truths that, the value of food for purposes 

 of growth depends not on the quantity of the various 



