convictions. Among the things thus brought more comfortably home to
us was the principle underlying longevity. It became apparent why
some living beings should live longer than others, and how any race
must be treated whose longevity it is desired to increase. Hitherto
we had known that an elephant was a long-lived animal and a fly
short-lived, but we could give no reason why the one should live
longer than the other; that is to say, it did not follow in
immediate coherence with, or as intimately associated with, any
familiar principle that an animal which is late in the full
development of its reproductive system will tend to live longer than
one which reproduces early. If the theory of "Life and Habit" be
admitted, the fact of a slow-growing animal being in general longer
lived than a quick developer is seen to be connected with, and to
follow as a matter of course from, the fact of our being able to
remember anything at all, and all the well-known traits of memory,
as observed where we can best take note of them, are perceived to be
reproduced with singular fidelity in the development of an animal
from its embryonic stages to maturity.

Take this view, and the very general sterility of hybrids from being
a CRUX of the theory of descent becomes a stronghold of defence. It
appears as part of the same story as the benefit derived from
judicious, and the mischief from injudicious, crossing; and this, in
its turn, is seen as part of the same story, as the good we get from
change of air and scene when we are overworked. I will not amplify;
but reversion to long-lost, or feral, characteristics, the phenomena
of old age, the fact of the reproductive system being generally the
last to arrive at maturity--few further developments occurring in
any organism after this has been attained--the sterility of many
animals in confinement, the development in both males and females