oportion to its increased action; that while generating a modified
_consensus_ of functions and of structures, the activities are at the
same time impressing this modified _consensus_ on the sperm-cells and
germ-cells whence future individuals are to be produced; and that in
ways mostly too small to be identified, but occasionally in more
conspicuous ways and in the course of generations, the resulting
modifications of one or other kind show themselves. Further, it seems to
me that as there are certain extensive classes of phenomena which are
inexplicable if we assume the inheritance of fortuitous variations to be
the sole factor, but which become at once explicable if we admit the
inheritance of functionally-produced changes, we are justified in
concluding that this inheritance of functionally-produced changes has
been not simply a co-operating factor in organic evolution, but has been
a co-operating factor without which organic evolution, in its higher
forms at any rate, could never have taken place.

Be this or be it not a warrantable conclusion, there is, I think, good
reason for a provisional acceptance of the hypothesis that the effects
of use and disuse are inheritable; and for a methodic pursuit of
inquiries with the view of either establishing it or disproving it. It
seems scarcely reasonable to accept without clear demonstration, the
belief that while a trivial difference of structure arising
spontaneously is transmissible, a massive difference of structure,
maintained generation after generation by change of function, leaves no
trace in posterity. Considering that unquestionably the modification of
structure by function is a _vera causa_, in so far as concerns the
individual; and considering the number of facts which so competent an
observer as Mr. Darwin regarded as evidence that transmission of such
modifications takes place in particular cases; the hypothesis that such
transmission takes place in conformity with a general law, holding of
all active structures, should, I think, be regarded as at least a good
working hypothesis.

* * * * *

But now supposing the broad conclusion above drawn to be
granted--supposing all to agree that from the beginning, along with
inheritance of useful variations fortuitously arising, there has been
inheritance of effects produced by use and disuse; do there remain no
classes of organic phenomena unaccounted for? To this question I think
it m