MAXIMUM, MINIMUM, AND OPTIMUM. 43 



tional activity of the individual organs, to the size of the animal, 

 and also, as I must now add, to its special adaptation to a certain 

 mode of life. It is clear that a large animal must consume 

 actually more food than a small one, hut with relation to the 

 mass of the animal the proportion may be precisely inverse. 

 Thus we know that a caterpillar, at the period of its most 

 rapid growth, eats a great deal more in proportion than a dog 

 or an elephant. The determination of the absolute and relative 

 amount of nourishment needed by different kinds of animals 

 is extremely difficult and of no importance to the present 

 enquiry. It offers, indeed, only two points of more general 

 interest, of which one shall at once come under discussion, while 

 the other the relative amount of nourishment required by 

 carnivorous and herbivorous animals will be treated later. 



The amount of daily nourishment needed differs very widely 

 in individuals of the same species ; one will eat, another will 

 drink, more than others ; but they will all be apparently 

 equally thriving, excepting in cases of actual over-eating or 

 privation. Between these two extremes which both result in 

 death, because the maximum of utilised nutrition is exceeded or 

 the minimum is not attained there is a graduated scale of 

 quantities, which are less and less favourable as they approach 

 these dangerous extremes. Hence a point must exist between 

 the two, which is the most favourable as regards the mass of 

 food introduced into the stomach. This may be briefly desig- 

 nated as the optimum of food. But this optimum does not 

 lie, as it might be supposed that it should, exactly halfway 

 between the two extremes, but may lie, according to the 

 creature's needs, nearer to the one or the other. It is, of course, 

 of the highest interest to ascertain what the optimum of daily 

 nourishment is for different animals, since this must be one of 

 the most potent influences which govern the constantly vary- 

 ing numbers of species and individuals. Unfortunately no data 

 of general value exist on this point. 10 We know, with tolerable 

 accuraqy, the optimum of nutrition for man, for the domestic 

 animals, and for those that have been subjected to physiological 

 experiment ; also for some others, such as many birds and 

 insects, which are of interest to the husbandman. But this 



