There is an old and persistent tendency in the human mind to conceive of the causal relation as rationally explanatory, and therefore to assimilate it, vaguely or explicitly, to the logical relations of inclusion, implication, or equivalence. That ‘ there cannot be more in the effect than there is in the cause’ is one of the propositions that men have been readiest to accept as axiomatic; a cause, it has been supposed, does not ‘ account for ‘ its effect, unless the effect is a thing which the eye of reason could somehow discern in the cause, upon a sufficiently thorough analysis. This antipathy to the notion of an absolute epigenesis has left its effect, unless the effect is a thing which the eye of reason could somehow discern in the cause, upon a sufficiently thorough analysis. This antipathy to the notion of an absolute epigenesis has left its mark deep and wide upon the history of thought; it appears, indeed, at the very outset of Western speculation in the struggles of the physiologers with the supposed difficulty of admitting qualitative change. Two of the later phases of what may be named the preformationist assumption about causality may pertinently be remembered here.