protested against the supposition that functionally produced
modifications were an adequate explanation of all the phenomena of
organic modification. He declares accident and the chances and
changes of this mortal life to be potent and frequent causes of
variations, which, being not infrequently inherited, result in the
formation of varieties and even species, but considers these causes
if taken alone as no less insufficient to account for observable
facts than the theory of functionally produced modifications would
be if not supplemented by inheritance of so-called fortuitous, or
spontaneous variations. The difference between Dr. Erasmus Darwin
and Mr. Spencer does not consist in the denial by the first, that a
variety which happens, no matter how accidentally, to have varied in
a way that enables it to comply more fully and readily with the
conditions of its existence, is likely to live longer and leave more
offspring than one less favoured; nor in the denial by the second of
the inheritance and accumulation of functionally produced
modifications; but in the amount of stress which they respectively
lay on the relative importance of the two great factors of organic
evolution, the existence of which they are alike ready to admit.

With Erasmus Darwin there is indeed luck, and luck has had a great
deal to do with organic modification, but no amount of luck would
have done unless cunning had known how to take advantage of it;
whereas if cunning be given, a very little luck at a time will
accumulate in the course of ages and become a mighty heap. Cunning,
therefore, is the factor on which, having regard to the usage of
language and the necessity for simplifying facts, he thinks it most
proper to insist. Surely this is as near as may be the opinion
which common consent ascribes to Mr. Spencer himself. It is
certainly the one which, in supporting Erasmus Darwin's system as