412 LAWS OP MULTIPLICATION. 



differences of fertility have the same general interpretation. 

 If the small variations of structure and function that arise 

 within the limits of each species, are due to actions like those 

 which, by their long-accumulating effects, have produced the 

 immense contrasts between the various types; we may con- 

 clude that, similarly, the actions to which changes in the 

 rate of multiplication of each species are due, also produce, 

 in great periods of time, the enormous differences between 

 the rates of multiplication of different species. 



Before inquiring in what ways the rapidities of increase 

 are adjusted to the requirements, both temporary and perma- 

 nent, it will be needful to look at the factors. Let us set 

 down first those which belong to the environment, and then 

 those which belong to the organism. 



316. Every living aggregate being one of which the 

 inner actions are adjusted to balance outer actions, it follows 

 that the maintenance of its moving equilibrium depends on 

 its exposure to the right amounts of these actions. Its 

 moving equilibrium may be overturned if one of these actions 

 is either too great or too small in amount ; and it may be so 

 overturned either by excess or defect of some inorganic 

 agency in its environment, or by excess or defect of some 

 organic agency. 



Thus a plant, constitutionally fitted to a certain warmth 

 and humidity, is killed by extremes of temperature, as well 

 as by extremes of drought and moisture. It may dwindle 

 away from want of soil, or die from the presence of too great 

 or too small a quantity of some mineral substance which the 

 soil supplies to it. In like manner, every animal can main- 

 tain the balance of its functions so long only as the environ- 

 ment adds to or deducts from its heat at rates not exceeding 

 definite limits. Water, too, must be accessible in amount 

 sufficient to compensate loss. If the parched air is rapidly 

 abstracting its liquid which there is no pool or river to 

 restore, its functions cease; and if it is an aquatic creature, 



