Such appears to be, at least in part, the machinery of heredity.  This understanding makes the germ substance perpetual and continuous, and explains why successive generations are alike.  It does not explain, indeed, why an individual inherits from its parents, but why it is like its parents.  While biologists are still in dispute over many problems connected with heredity, all are agreed to-day that this principle of the continuity of the heredity substance must be the basis of all attempts to understand the machinery of heredity.  But plainly this whole process is a function of the cell machinery.  While, therefore, the idea of the continuity of germ substance greatly simplifies our problem, we must acknowledge that once more we are thrown back upon the mysteries of the cell.  Until we can more fully explain the cell machine we must recognize our inability to solve the fundamental question of why an individual is like its parents.

[Illustration:  FIG. 50.—­Diagram illustrating the principle of heredity.

A represents an egg of a starfish.  From one half, the unshaded portion, develops the starfish of the next generation, B.  The other is distributed without change in the ovaries, ov, of the individual, B.  From these ovaries arises the next egg, A’, with its germ plasm.  This germ plasm is evidently identical with that in A, since it is merely a bit of the same handed down through the individual, B.  In the development of the next generation the process is repeated, and hence B’ will be like B, and the third generation of eggs identical with the first and second.  The undifferentiated part of the germ plasm is thus simply handed on from one generation to the next.]

But plainly reproduction and heredity, as we have thus far considered them, will be unable to account for the slow modification of the machine; for in accordance with the facts thus far outlined, each generation would be precisely like the last, and there would be no chance for development and change from generation to generation.  If the individual is simply the unfolding of the powers possessed by a bit of germ plasm, and if this germ plasm is simply handed on from generation to generation, the successive generations must of necessity be identical.  But the living machine has been built by changes in the successive generation, and hence plainly some other factor is needed.  This factor is variation.

==Variation.==—­Variation is the principle that produces modification of type.  Heredity, as just explained, would make all generations alike.  But nothing is more certain than that they are not alike.  The fact of variation is patent on every side, for no two individuals are alike.  Successive generations differ from each other in one respect or another.  Birds vary in the length of their bills or toes; butterflies, in their colours; dogs, in their size and shape and markings; and so on through an endless category.  Plants and animals alike throughout nature show variations in the greatest profusion.  It is these variations which must furnish us with the foundation of the changes which have gradually built up the living machine.