that each individual does not come from germ material identical with that from which either parent came, but from some of this material mixed with a similar amount from a different parent.  Now, the two parents are never exactly alike, and hence the germ plasm which each contributes to the offspring will not be exactly alike.  The offspring will thus be the result of the unfolding of a bit of germ plasm which will be different from that from which either of its parents developed, and these differences will result in congenital variations.  Sexual reproduction thus results in congenital variations; and if congenital variations are necessary for the evolution of the living machine—­and we shall soon see reason for believing that they are—­we find that sexual reproduction is a device adopted for bringing out such congenital variations.

==Inheritance of Variations.==—­The reason why congenital variations are needed for the evolution of the living machine is clear enough.  Evanescent variations can have no effect upon this machine, for they would disappear with the individual in which they appeared.  In order that they should have any influence in the process of machine building they must be permanent ones; or, in other words, they must be inherited from generation to generation.  Only as such variations are transmitted by heredity can they be added to the structure of the developing machine.  Therefore we must ask whether the variations are inherited.

In regard to the congenital variations there can be no difficulty.  The very fact that they are congenital shows us that they have been produced by variations in the germ plasm, and as such they must be transmitted, not only to the next generation, but to all following generations, until the germ plasm becomes again modified.  This germ plasm is handed on from generation to generation with all its variations, and hence the variations will be added permanently to the machine.  Congenital variations are thus a means for permanently modifying the organism, and by their agency must we in large measure believe that evolution through the ages has taken place.

With the acquired variations the matter stands quite differently.  We can readily understand how influences surrounding an animal may affect its organs.  The increase in the size of the muscles of the blacksmith’s arm by use we understand readily enough.  But with our understanding of the machinery of heredity we can not see how such an effect can extend to the next generation.  It is only the organ directly affected that is modified by external conditions.  Acquired variations will appear in the part of the body influenced by the changed conditions.  But the germ plasm within the reproductive glands is not, so far as we can see, subject to the influence of an increased use, for example, in the arm muscles.  The germ material is derived from the parents, and, if it is simply stored in the individual, how could