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 of ‘plasma, why should not the entire evolutionary 
process have also been the outcome of developmental inner 
causes? He 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. Logically the difference is unimpor- 
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. 
