46. DEVELOPMENT AND HEREDITY. 



beef; fish ; coagulated albumen of hen's egg; yellow 

 of hen's egg ; albuminoid substance of frog's egg 

 with liquid albumen of hen's &gg, substituted later 

 in the experiment; vegetable substances (algae). 

 3. A purely vegetable diet is insufficient to transform 

 the tadpole into the frog." These experiments afford 

 conclusive evidence that food is not merely the 

 source of supply of raw energy to a developing 

 organism ; and also that the innate forces in the 

 organism do not wholly control its development. 

 Food is one of the stimulating forces which guide 

 and determine the developmental reactions of the 

 organism. 



The effects of heat on life and development are so 

 marked and the universal necessity for its presence 

 so well known, as for example, in hatching eggs and 

 germinating seeds and sustaining all life, that this 

 part of the forces of the environment needs but a 

 bare mention. Heat and light are original sources of 

 almost all of the energy which displays itself in the 

 structure and movements of living things.^ The 

 peculiar effects of light upon animals, however, have 



1 Professor Guyot has expressed a " Law of Development and Life " 

 as follows: "Throughout the entire realm of nature, in the animal 

 world as well as in the vegetable, the development of life increases in 

 energy, and in the variety and perfection of the types, with the increas- 

 ing intensity of light and heat, from the poles to the equator." — 

 Guyot' s Physical Geography. 



