ch. vn.] SOURCES OF TERRESTRIAL ENERGY. 411 



vegetables. Herbivorous animals in each of the great classes, 

 feed directly upon vegetable fibre, and so rearrange its mole- 

 cules that the resultant tissues are more highly nitrogenous 

 than those from which they were formed. More active car- 

 nivorous animals derive from the enormous chemism latent 

 in these nitrogenous fabrics the vital energy displayed in 

 their rapid bounds and in their formidable grip. But the 

 energies which imprisoned this tremendous chemical force in 

 the complex molecules which the animal assimilates, were at 

 first supplied by sunbeams. Metamorphosed originally into 

 the static energy of vegetable tissue, this sun-derived power 

 is again metamorphosed into the dynamic energy which main- 

 tains the growth of the animal organism. And from the 

 same primeval source comes the surplus energy, which after 

 the demands of growth or repair have been satisfied, is ex- 

 pended in running, jumping, flying, swimming, or climbing, 

 as well as in fighting with enemies and in seizing and de- 

 vouring prey. 



Besides these indirect and doubly-indirect methods in which 

 animals differentiate solar energy, there are ways in which 

 the metamorphosis is directly effected. To cite Dr. Carpen- 

 ter's conclusions, as epitomized by Mr. Spencer : — " The 

 transformation of the unorganized contents of an egg into 

 the organized chick, is altogether a question of heat : with- 

 hold heat and the process does not commence ; supply heat 

 and it goes on while the temperature is maintained, but 

 ceases when the egg is allowed to cool. ... In the meta- 

 morphoses of insects we may discern parallel facts. Experi- 

 ments show not only that the hatching of their eggs is deter- 

 mined by temperature, but also that the evolution of the 

 pupa into the imago is similarly determined ; and may be im- 

 mensely accelerated or retarded according as heat is artificially 

 bupplied or withheld." The phenomena thus briefly cited 

 are to be classed under the general head of organic stimulus ; 

 uni in a wide sense, one might almost say that all stimulus 



