110 Mr. H. G. Set'ley on the Laws tv/iich have determined 



Translated into other language, this means that since the snn 

 is the chief appreciable source of energy on the earth, without 

 which little or no motion would be manifested, it follows that 

 organisms are storehouses in which the sun^s energy has been 

 accumulated in the form of work, and therefore that what are 

 called grades of organization in classification are only ways of 

 expressing the different degrees of energy that organic struc- 

 tures have stored up. 



17. The manifestation of life on the earth is in every way 

 most abundant in the tropics, plentiful in the temperate zone, 

 and poor in genera and poor in species near the frigid poles of 

 the earth. The exuberance of life, whether in individuals or 

 species, over the whole earth, or upon one district in different 

 seasons, coincides with the preponderance of heat. Heat acts 

 indirectly for the most part; for when applied to an egg, it is 

 partly converted into motion, causing the particles of the egi^ 

 to move, and it enables them to enter into new chemical com- 

 binations, differentiating parts until the entire organism is 

 formed. The energy which differentiates the individual egg is 

 gi ciitest at the tropics, where the differentiation of life is greatest. 



18. Every organism is subject to two series of modifying 

 agents: — 1st, the external changes produced by the stinndus of 

 the circumstances of existence; 2ndly, the chemical changes 

 set up by contact of food with the viscera. Both of these 

 sources contribute energy. When a mammal, for instance, 

 moves, its work in part takes the form of motion ; but the suc- 

 cession of falls which constitute walking or running convert a 

 part of that motion into heat ; this heat induces an expansion of 

 the structures, enabling the nutritive fluid to permeate and cir- 

 culate more rapidly, nourishing most the structures most used. 

 Hence the development of parts with use. The development of 

 the skeleton is chiefly due to differentiation of external func- 

 tions ; the development of the viscera is chiefly due to different 

 functions imposed by food. 



The viscera, therefore, are more liable to vary than the mus- 

 cles ; but their variation depends on the power of muscle and 

 nerve in obtaining food. Therefore external changes are accom- 

 panied by internal changes. And since changes are inherited, 

 they accumulate. 



19. The individual being only liable to motion over so limited 

 an area as to be practically fixed, yet experiences some results 

 of enormous migrations from the change of seasons. 



20. Since the heat of the earth may be assumed to be distri- 

 buted approximately in zoned gradations of latitude, it will fol- 

 low, from the preceding considerations, that if species were left 

 to themselves for ever, the most highly organized would be at 



