If there were no ancestry, there would be no posterity. But to answer the question _why_ certain of the posterity depart from the rule which others follow, we must know, not the ancestry, but how it came about _either_ that at a certain moment a certain gamete divided from its fellows in a special and unwonted fashion; _or_, though the words are in part tautological, the reason why the union of two particular gametes in fertilisation took place in such a way that gametes having new specific properties resulted[161]. No one yet knows how to use the facts of ancestry for the elucidation of these questions, or how to get from them a truth more precise than that contained in the statement that a diversity of specific consequences (in heredity) may follow an apparently single specific disturbance. Rarely even can we see so much. The appeal to ancestry, as introduced by Professor Weldon, masks the difficulty he dare not face. [161] May there be a connection between the extraordinary fertility and success of the _Telephone_ group of peas, and the peculiar frequency of a blended or mosaic condition of their allelomorphs? The conjecture may be wild, but it is not impossible that the two phenomena may be interdependent. In other words, it is the _cause of variation_ we are here seeking. To attack that problem no one has yet shown the way. Knowledge of a different order is wanted for that task; and a compilation of ancestry, valuable as the exercise may be, does not provide that particular kind of knowledge. Of course when once we have discovered by experiment that--say, _Telephone_--manifests a peculiar behaviour in heredity, we can perhaps make certain forecasts regarding it with fair correctness; but that any given race or individual will behave in such a way, is a fact not deducible from its ancestry, for the simple reason that organisms of identical ancestry may behave in wholly distinct, though often definite, ways. It is from this hitherto hopeless paradox that Mendel has begun at last to deliver us. The appeal to ancestry is a substitution of darkness for light. VII. THE QUESTION OF ABSOLUTE PURITY OF GERM-CELLS. But let us go back to the cases of defective “purity” and consider how the laws of ancestry stand in regard to them. It appears from the facts almost certain that purity may sometimes be wanting in a character which elsewhere usually manifests it. Here we approach a question of greater theoretical consequence to the right apprehension of the part borne by Mendelian principles in the physiology of heredity. We have to consider the question whether the purity of the gametes in respect of one or other antagonistic character is or is likely to be in case of _any_ given character a _universal_ truth? The answer is unquestionably--No--but for reasons in which “ancestry” plays no part[162]. [162] This discussion leaves “false hybridism” for separate consideration.