Thirdly, the introduction of slightly new elements into a method already established varies it beneficially; the new is soon fused with the old, and the monotony ceases to be oppressive.  But if the new be too foreign, we cannot fuse the old and the new—­nature seeming to hate equally too wide a deviation from ordinary practice and none at all.  This fact reappears in heredity as the beneficial effects of occasional crossing on the one hand, and on the other, in the generally observed sterility of hybrids.  If heredity be an affair of memory, how can an embryo, say of a mule, be expected to build up a mule on the strength of but two mule-memories?  Hybridism causes a fault in the chain of memory, and it is to this cause that the usual sterility of hybrids must be referred.

Fourthly, it requires many repeated impressions to fix a method firmly, but when it has been engrained into us we cease to have much recollection of the manner in which it came to be so, or indeed of any individual repetition, but sometimes a single impression if prolonged as well as profound, produces a lasting impression and is liable to return with sudden force, and then to go on returning to us at intervals.  As a general rule, however, abnormal impressions cannot long hold their own against the overwhelming preponderance of normal authority.  This appears in heredity as the normal non-inheritance of mutilations on the one hand, and on the other as their occasional inheritance in the case of injuries followed by disease.

Fifthly, if heredity and memory are essentially the same, we should expect that no animal would develop new structures of importance after the age at which its species begins ordinarily to continue its race; for we cannot suppose offspring to remember anything that happens to the parent subsequently to the parent’s ceasing to contain the offspring within itself.  From the average age, therefore, of reproduction, offspring should cease to have any further steady, continuous memory to fall back upon; what memory there is should be full of faults, and as such unreliable.  An organism ought to develop as long as it is backed by memory—­that is to say, until the average age at which reproduction begins; it should then continue to go for a time on the impetus already received, and should eventually decay through failure of any memory to support it, and tell it what to do.  This corresponds absolutely with what we observe in organisms generally, and explains, on the one hand, why the age of puberty marks the beginning of completed development—­a riddle hitherto not only unexplained but, so far as I have seen, unasked; it explains, on the other hand, the phenomena of old age—­hitherto without even attempt at explanation.