A cause that permanently 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. Nägeli 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 successive stage follows with mechanical necessity from the preceding. 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 individual 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 unimportant. 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. Nägeli discusses a question in this connection, which, he says, has been unnecessarily confused in the descent theory. Since we are entirely in the dark as to how much time has been required for the formation of phyla, so also are we ignorant as to how long it may have taken for each step in advance. We may err equally in ascribing too much and too little time to the process. It is, moreover, not necessary that for every step the same amount of time should have been required. On the contrary, the probability is that recognizable changes may at times follow each other rapidly, and then for a time come to a standstill,—just as in the development of the individual there are periods of more rapid and others of less rapid change.