THE FACTORS OF ORGANIC EVOLUTION. 115 
ample, Professor Cope argues very ably that bones are 
lengthened by both stretch and impact, and that modifi- 
cations thus produced are inherited. Even granting 
that this is true, how would it be possible for this pro- 
cess of lengthening to cease, since in active animals the 
stretch and impact must be continual? Professor Cope 
answers that the growth ceases when “ equilibrium ”’ is 
reached. I confess I can not understand this explana- 
tion, since the assumed stimulus to growth must be con- 
tinual. But, granting again that growth may stop when 
an animal’s legs become long enough to “satisfy its 
needs,” how on this principle are we to account for the 
shortening of legs, as, for example, in the turnspit dog 
and the ancon sheep and numberless cases occurring in 
Nature? If any one species was able, by taking thought 
of mechanical stresses and strains, to add one cubit unto 
its stature, how could the same stresses and strains be 
invoked to decrease its stature? 
These evidences are, I know, not the strongest ones 
which can be adduced in support of the Lamarckian 
factors. There are at present a relatively small num- 
ber of such arguments which seem to be valid and the 
great force of which I fully admit. But the cases which 
I have cited are, I believe, fair samples of the majority 
of the evidences so far presented, and in the face of 
such “evidence” it is not surprising that one who is 
himself a profound student of the subject and a con- 
vinced Lamarckian prays that the Lamarckian theory 
may be delivered from its friends.* 
6. Another line of evidence, and by far the most 
promising, is that of direct experiment. So far, most 
of the experiments which have been carried on to deter- 
mine this question have been carried only halfway to a 
*H. F. Osborn. Evolution and Heredity. Biological Lec- 
tures, 1890. 
