External and Internal Factors in Evolution 333 



the common broad undivided leaves. Many such examples 

 have been recorded which can only be explained by assuming 

 that a cell, or a group of cells, like those from which the 

 other branches arose, have become changed in some unknown 

 way as the result of inner causes. The properties that are 

 permanent and inherited are contained in the idioplasm, 

 which the parent transmits to its offspring. A cause^that per- 

 manently transforms the organism must also transform the 

 idioplasm. How powerless, in. comparison to internal causes, 

 the external causes are is shown most conclusively in grafting. 

 The graft, although it receives its nourishment through the 

 stock, which may be another species, remains itself unchanged. 



Nageli makes the following interesting comparison 

 between the development of the individual from an egg, and 

 the evolution, or development, of the phylum. No one will 

 doubt that the egg during the entire time of its process of 

 transformation is guided by internal factors. Each succes- 

 sive stage follows with mechanical necessity from the pre- 

 ceding. If an animal can develop from inner causes from 

 a drop_.oi_plasma, why shoul d no t th e .entire .evolutionary 

 process have also been the outeome-ef-develepmentaljnner 

 causes ? <Qe admits that there is a difference in the two 

 cases in that the plasma that forms the egg has come from 

 another animal, and contains all the properties of the indi- 

 vidual in a primordial condition. In the other case we must 

 suppose that the original drop of plasma did not contain at 

 first the primordium of definite structures, but only the 

 ability to form such. Logicall y the difference is ummpor- 

 tant. The main point is that in the primordium of the germ 

 a special peculiarity of the substance is present which by 

 forming new substances grows, and changes as it grows, and 

 the one change of necessity excites the next until finally a 

 highly organized being is the result. 



Nageli discusses a question in this connection, which, he 

 says, has been unnecessarily confused in the descent theory. 



