18 TERRESTRIAL ADAPTATIONS. 



form ; thus solar and terrestrial heat, air, mois- 

 ture, and probably many other apparently con- 

 flicting agents, join to produce our weather, 

 which never deviates very far from a certain 

 average standard. 



Now a general fact, which we shall endeavour 

 to exemplify in the following chapters, is this : 

 That those properties of plants and animals 

 which have reference to agencies of a peri- 

 odical character, have also by their nature a peri- 

 odical mode of working ; while those properties 

 which refer to agencies of constant intensity, 

 are adjusted to this constant intensity : and 

 again, there are peculiarities in the nature of 

 organized beings which have reference to a 

 variety in the conditions of the external world, 

 as, for instance, the difference of the organized 

 population of different regions : and there are 

 other peculiarities which have a reference to the 

 constancy of the average of such conditions, and 

 the limited range of the deviations from that 

 average; as for example, that constitution by 

 which each plant and animal is fitted to exist 

 and prosper in its usual place in the world. 



And not only is there this general agreement 

 between the nature of the laws which govern the 

 organic and inorganic world, but also there is a 

 coincidence between the arbitrary magnitudes 

 which such laws involve on the one hand and on 

 the other. Plants and animals have, in their 



