16 DEVELOPMENT AND HEREDITY. 



Stimuli withheld from a growing animal, the same 

 development would fail to occur. In this case, 

 therefore, although we are unable to analyse the 

 details of the process, we have, without doubt, a 

 certain amount of development directly caused by 

 forces of the environment acting upon the animal. 



But the amount of development which we can 

 ascribe to causes acting thus directly from the 

 environment, seems small when compared with that 

 which takes place during the embryonic period of 

 the life of an animal. At first sight, this latter 

 appears to be of an entirely different nature. It 

 seems to be spontaneous. But we know that in the 

 embryonic life, as well as in later life, the changes 

 in material structure are only effects caused 

 according to physical laws : therefore the nature 

 of embryonic development cannot differ from that 

 of the adult, except in the origin of the forces which 

 cause the activity. In each case, the movements of 

 the particles in building up the organism must 

 proceed according to physical and chemical laws. 



We cannot follow the different steps of devel- 

 opment of the nervous system in the embryo by 

 observing the correlative psychical activity; but 

 when in the normal embryological development of 

 an animal we see marked physical changes taking 

 place in the brain, must we not infer that they 



