Our next problem must be to search for an explanation of these variations.  With the acquired variations we have no particular trouble, for they are easily explained as due to the direct action of the environment upon animals.  One of the fundamental characters of the living protoplasm (using the word now in its widest sense) is its extreme instability.  So unstable is it that any disturbing influence will affect it.  If two similar unicellular organisms are placed under different conditions they become unlike, since their unstable protoplasm is directly affected by the surrounding conditions.  With higher animals the process is naturally a little more complicated; but here, too, they are easily understood as part of the function of the machine.  One of the adjustments of the machine is such that when any organ is used more than usual the whole machine reacts in such a way as to send more blood to this special organ.  The result is a change in the nutrition of the organ and a corresponding variation in the individual.  Thus acquired variations are simply functions of the action of the machine.

Congenital variations, however, can not receive such an explanation.  Being born with the individual, they can not be produced by conditions affecting him, but rather to something affecting the germ plasm from which he sprung.  The nature of the germ plasm controls the nature of the individual, and congenital variations must consequently be due to its variations.  But it is not so easy to see how this germ plasm can undergo variation.  The conditions which surround the individual would affect its body, but it is not easy to believe that they would affect the germinal substance.  Indeed, it is not easy to see how any external conditions can have influence upon this germinal material if it is not an active part of the body, but is simply stored within it for future use in reproduction.  How could any changes in the environment of the individual have any effect upon this dormant material stored within it?  But if we are correct in regarding this germ material in the reproductive bodies as the basis of heredity and the guiding force in development, then it follows that the only way in which congenital variations can occur is by some variations in the germ plasm.  If a child developed from germ plasm identical with that from which its parents developed, it would inherit identical characters; and if there are any congenital variations from its parents, they must be due to some variations in the germ plasm.  In other words, in order to explain congenital variations we must account for variations in the germ plasm.