6Z IOWA ACADEMY OF SCIENCES. 



upon as resulting in the production of slight variations 

 from the type, rather than as activelj^ co-operating with 

 heredity to bring about a typical development. 



In strong opposition to this point of view are the epi- 

 genetic theories of heredity. These attribute far more 

 importance, throughout the whole period of development, 

 to the external factor, environment. The germ, like all 

 other living matter, is of course regarded as of extremely 

 complex organization, but the various elements composing 

 it in no way represent corresponding elements of the adult 

 body into which it is to develop. We have a mass of 

 protoplasm, capable of growth by the assimilation of new 

 material from the outside; responding during that growth, 

 however, to all the various kinds of stimuli from the 

 environment, — heat, light, electricity, chemism, moisture, 

 oxygen, molar impulses, adhesive force, pressure, etc. 

 Even supposing the protoplasm constant in structure, 

 changes in these various forces would produce different 

 reactions. So long as the environment does not vary too 

 much from the normal, some reaction may be looked for; 

 but if too great a variation occurs development will cease. 

 But the protoplasm of the germs of different species may 

 differ much in structure, and consequently in the exact 

 response that will be made even to the same environ- 

 mental forces; in other words, the protoplasm exerts a 

 selective power not only on the elements of food to be 

 assimilated, but also on all the other external forces; thus 

 different germs respond differently even in identical envi- 

 ronments. The first growth and change of form, how- 

 ever, puts the organism into a new and different relation 

 to the environment, in which the external forces will pro- 

 duce necessarily different results; and so every successive 

 stage in development will have its own peculiar relation 

 to the environment, and make its own peculiar responses, 

 even though that environment remains constant. But 

 while the organism reacts to the environment, so it also 

 reacts upon it so as to modify it to some extent; also while 

 the organism reacts to the environment by undergoing 

 internal modifications of function, it also reacts to it by 



