THE INDISPENSABLE OPTIMUM. 65 



case the structure of the animal and of its organs might remain 

 unaltered in spite of the alteration in the nutrition, as, for 

 instance, seems to have been the case with the Coypu, those of 

 the Ohonos islands not differing in any way, so far as we know, 

 from their congeners on the mainland. But, finally, this 

 change in the food might have altered the structure of some 

 organs, particularly of those most directly interested, so far as 

 to make these changes conspicuous ; and a direct modifying 

 influence exercised by the conditions of existence afforded by 

 the food would be thereby proved. 



The direct modifying influence of food. — It is universally 

 known, and has never been denied, that the amount of food 

 exerts a very decided influence in determining the growth of 

 the individual and of its organs, as well as on its whole size ; 

 but this has oftsn been tendered as a means of explanation in 

 certain cases which have not been submitted to careful investiga- 

 tion. It can never, of course, be disputed that an animal must 

 take up the optimum of daily nutrition, which constantly varies 

 with its advancing age, in order to attain its normal size ; we 

 might even declare our opinion that very many living or 

 extinct animals might have grown to a size far beyond that 

 which they have in fact attained, if they had had more abundant 

 supplies of food at their disposal ; but it would be in the 

 highest degree illogical to assume, on the contrary, without 

 any experimental proof — as is unfortunately almost universally 

 done^that the small size of any particular animal in any 

 particular locality is invariably induced by a deficiency in the 

 food attainable there, where the optimum is seldom or never 

 attained. In these cases, as in all others, in consequence of the 

 extreme complication of the animal body and of its functions, 

 the same effect may be produced in many different ways. 



But the amount of food attainable affects not merely the 

 size of the animal, but also determines, and even modifies, 

 certain vital functions. It is self-evident that an optimum 

 of nutrition can alone insure the normal functions of all the 

 organs ; if it does not attain the optimum, the functional activity 

 of all the organs is impaired ; modifications at the same time 

 occur in their structure, i.e. the animals grow leaner, become 



