Now, there are two methods by which we may suppose that these variations in the germ may arise.  The first is by the direct influence upon the germ plasm of certain unknown external conditions.  The life substance of organisms is always very unstable, and, as we have seen, acquired variations are caused by external influences directly affecting it.  Now, the hereditary material is also life substance, and it is plainly a possibility for us to imagine that this germ material is also subject to influences from the conditions surrounding it.  That such variations do occur appears to be hardly doubtful, although we do not know what sort of influences can produce them.  If the germ plasm is wholly stored within the reproductive gland, it is certainly in a position to be only slightly affected by surrounding conditions which affect the animal.  We can readily understand that the use of an organ like the arm will affect it in such a way as to produce changes in its protoplasm, but we can hardly imagine that such use of the arm would produce any change in the hereditary substance which is stored in the reproductive organs.  External conditions may thus readily affect the body, but not so readily the germ material.  Even if such material is distributed more or less over the body instead of being confined to the reproductive glands, as some believe, the difficulty is hardly lessened.  This difficulty of understanding how the germ plasm can be affected by external conditions has led one school of biologists to deny that it is subject to any variation by external conditions, and hence that all modification of the germ plasm must come from some other source.  Probably no one, however, holds this position to-day, and it is the general belief that the germ plasm may be to some slight extent modified by external conditions.  Of course, if such variations do occur in the germ plasm they will become congenital variations of the next generation, since the next generation is the unfolding of the germ plasm.

The second method by which the variations of germ plasm may arise is apparently of more importance.  It is based upon the fact that, with all higher animals and plants at least, each individual has two parents instead of one.  In our study of cells we have seen that the machinery of the cell is such that it requires in the ordinary process of reproduction the union of germinal material from two different individuals to produce a cell which can develop into a new individual.  As we have seen, the egg gets rid of half its chromosomes in order to receive an equal number from a male parent; and thus the fertilized egg contains chromosomes, and hence hereditary material, from two different individuals.  Now, this sexual reproduction occurs very widely in the organic world.  Among some of the lowest forms of unicellular organisms it is not known, but in most others some form of such union is universal.  Now, here is plainly an abundant opportunity for congenital variations; for it is seen