Introduction 15 
I showed that if the view for which I was contending 
was taken, many facts which, though familiar, were still 
without explanation or connection with our other ideas, 
would remain no longer isolated, but be seen at once as 
joined with the mainland of our most assured 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 immed- 
iate 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 any- 
thing 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 strong- 
hold 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 
