208 DEVELOPMENT AND HEREDITV. 



it the matter and energy requisite to its growth, 

 and stimulating its activity. These two sets of 

 causes produce the development of every individual, 

 and no change can occur in the development of 

 new individuals except through some change in the 

 causes. These two sets of causes, however, are 

 not independent of each other. The whole chain 

 of associations comprising the hereditary impulse 

 may be present in the germ, but there will be no 

 development unless certain deiinite external forces, 

 in their proper sequence, supply energy and matter 

 to the organism as it progresses in its growth. 



As the development of an organism is a process 

 of constant change, there must arise at every 

 moment a new relation of the forces which cause 

 the change. At each moment the action of the 

 various forces causes the developing organism to 

 change. This change results in a new arrange- 

 ment of the parts of the organism, i.e. a new 

 combination of its internal forces. Now, whether 

 the external forces remain the same, or whether 

 they change, we have in either case a new combi- 

 nation of the whole complex arrangement of forces 

 which act to produce development. This new 

 combination we must regard as the cause of the 

 immediately ensuing step in development. Thus 

 every step in development is the result of a partic- 



