348 NATURAL SCIENCE. May, 
shelter and nourish the reproductive elements can produce some kind 
of effect upon them, and though reproductive elements may persis- 
tently affect other reproductive elements, as in the case of Lord 
Morton’s mare, yet such and similar facts do not in the least decide 
the question whether the cells, or units, of the bodily structure can 
affect the reproductive substance, or germ-plasm, in such a way as to 
convey their particular acquired modifications, or changes, to 
posterity. 
A few words are also necessary in answer to the claim that since 
congenital characters are heritable, the onus of proving that acquired 
characters are “ not inheritable ” falls upon the Neo-Darwinian. The 
implication that the one case affects the other cannot be sustained. 
It is perfectly certain and perfectly well-known that there is a wide 
difference between the two classes of characters, and that they are 
not comparable in their degree of effect upon heredity—otherwise 
there would be no more dispute concerning the one class than con- 
cerning the other. No one questions the immediate and decisive 
heritability of congenital mutilations and malformations, or of any other 
congenital peculiarity, however striking or minute. On the other 
hand, it is a matter of general experience that the loss of a tooth, or 
eye, or limb, or finger-joint, is no more transmitted to offspring than 
a sunburnt complexion, or a clipped beard, or a knowledge of Latin 
or cookery. In proportion as the nature of each case admits of clear 
decisive proof, we see that acquired characters are not inherited ; 
no indisputable evidence to the contrary has ever been given. 
The presumption arising from the pvoven facts, therefore, is that non- 
inheritance of acquired characters is the general rule, and the onus 
probandi, therefore, rests with those who assert the contrary. We 
cannot prove a universal negative; and it is not easy to find or 
suggest absolutely decisive tests of the admittedly slight power of use- 
inheritance, or of the size-reducing power of panmixia. The effect of 
use-inheritance on offspring is imperceptible. Darwin acknowledges 
that several generations must elapse before any appreciable result 
follows ;'* and during this interval other factors would be at work. 
We perceive the apparently insuperable difficulty, or practical 
impossibility, of excluding all other factors in such delicate and com- 
plicated cases. But if the Neo-Lamarckian will accept his own cases, 
and allow us to adopt his own assumption that extraneous factors are 
adequately excluded, we can show that the effects of use and disuse 
are not inherited in some features just as conclusively (or inconclusively) 
as he shows that, in other respects, they are inherited. If, for instance, 
use-inheritance is proved by the shortening of the less used wing- 
bones of the domestic duck, or the leg-bones of the rabbit, then it is 
disproved by the thickening of these bones ; and, conversely, if it is 
proved by the marked thinness or lightness of the wing-bones of the 
M4 Variation of Animals and Plants under Domestication, vol. ii., p. 288. 
