The "direct proofs" of use-inheritance are not as plentiful as might be
desired, it appears (pp. 24-28). This acknowledged "lack of recognized
evidence" is indeed the weakest feature in the case, though Mr. Spencer
would fain attribute this lack of direct proof to insufficient
investigation and to the inconspicuous nature of the inheritance of the
modification. But there is an almost endless abundance of conspicuous
examples of the effects of use and disuse in the individual. How is it
that the subsequent inheritance of these effects has not been more
satisfactorily observed and investigated? Horse-breeders and others
could profit by such a tendency, and one cannot help suspecting that the
reason they ignore it must be its practical inefficacy, arising probably
from its weakness, its obscurity and uncertainty or its non-existence.