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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 farther 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.

Sixthly, those organisms that are the longest in reaching maturity should on the average be the longest-lived, for they will have received the most momentous impulse from the weight of memory behind them. This harmonises with the latest opinion as to the facts. In his article on Weismann in the Contemporary Review for May 1890, Mr. Romanes writes: “Professor Weismann has shown that there is throughout the metazoa a general correlation between the natural lifetime of individuals composing any given species, and the age at which they reach maturity or first become capable of procreation.” This, I believe, has been the conclusion generally arrived at by biologists for some years past.

Lateness, then, in the average age of reproduction appears to be the principle underlying longevity. There does not appear at first sight to be much connection between such distinct and apparently disconnected phenomena as 1, the orderly normal progress of development; 2, atavism and the resumption of feral characteristics; 3, the more ordinary resemblance inter se of nearer relatives; 4, the benefit of an occasional cross, and the usual sterility of hybrids; 5, the unconsciousness with which alike bodily development and ordinary physiological functions proceed, so long as they are normal; 6, the ordinary non-inheritance, but occasional inheritance of mutilations; 7, the fact that puberty indicates the approach of maturity; 8, the phenomena of middle life and old age; 9, the principle underlying longevity. These phenomena have no conceivable bearing on one another until heredity and memory are regarded as part of the same story. Identify these two things, and I know no phenomenon of heredity that does not immediately become infinitely more intelligible. Is it conceivable that a theory which harmonises so many facts hitherto regarded as without either connection or explanation should not deserve at any rate consideration from those who profess to take an interest in biology?