n 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