DECEMBER 17, 1897. ] 
to the accumulation of acquired variations. 
But variations acquired as a result of use 
and disuse are plainly never transmitted. 
Thus an infant’s limb never attains to the 
adult standard except in response to the 
same stimulation (exercise) as that which 
developed the parent’s limb. The same is 
true of all the other structures which in the 
parent underwent development as a result 
of use, or subsequent retrogression in the 
absence of it. These, like the limbs, do not 
develop or retrogress in the infant except as 
a result of similar causes. Plainly, then, 
what is transmitted to the infant is not the 
modification, but only the power of acquiring 
a under similar curcwmstances—a power which 
has undergone such an evolution in high 
animal organisms that, as I say, in man, 
for instance, almost all the development 
changes which occur between infancy and 
manhood are attributable toit. It follows, 
therefore, that the exquisite codrdination 
of all the parts of a high animal is not 
due to the inherited effects of use and dis- 
use, but to this great power of acquiring 
modifications along certain definite lines ; 
so that if an animal varies in such a way 
as to have one of its structures (¢. g., horn, 
a structure which is wholly inborn) larger 
than in the parent, then all the other struc- 
tures associated with it, owing to the 
increased strain (7. e., the increased stimu- 
lation) put on them, undergo a correspond- 
ing modification, and thus preserve the 
harmony of ail the parts of the whole. 
So alsoif the horn (for instance) be smaller 
than in the parent, the lesser strain placed 
by it on associated structures causes these 
also to develop less than in the parent, 
whereby again the harmony of the whole is 
preserved. 
I have dwelt at greater length on this 
neglected subject of acquired characters 
(properly so-called) elsewhere,* but I think 
I have said enough even here to demon- 
*Vide The Present Evolution of Man, pp. 108-21. 
SCIENCE. 
901 
strate its immense importance. The power 
of acquiring fit modifications in response to 
appropriate stimulation is that which 
especially differentiates high animal organ- 
isms from low animal organisms.* With- 
out this power and the plasticity which re- 
sults from it the multitudinous parts of 
high animals could not well be coordi- 
nated, and, therefore, without it their evo- 
lution could scarcely have been possible. 
Indeed, it is not too much to say, so vitally 
important is this power to the higher ani- 
mals, that, as regards them, the chief aim 
(if I may use the expression) of natural 
selection has been to evolve it. But, since 
this power of developing in response to the 
stimulation of use operates mainly along 
certain definite lines, which are not quite 
the same in every species, the different 
species differ as regards size and shape, not 
only in characters which are inborn, but 
also in those which are acquired. Thus an 
ox differs in size and shape from a man not 
alone in inborn characters, but also in 
characters which are acquired as a result 
of exercise and use. The structures of both 
the ox and man develop in response to ap- 
propriate stimulation, but not quite in the 
same direction, nor in the same proportion, 
nor to the same degree ; hence, to some ex- 
tent the differences in size and shape be- 
twixt the two animals. Consider, for in- 
stance, the hind limbs of the ox and man: 
in both these grow greatly as a response to 
the stimulation of exercise, but the lines of 
growth being somewhat different the limbs 
do not approximate in shape and _ size. 
Presently, when we consider mind, we 
shall realize even more strikingly the im- 
portance of our subject, and perceive how 
deeply it concerns many fields of thought 
and investigation which have greatly in- 
terested mankind in allages; but I have 
still something more to say as regards 
* The truth of this, as we shall see, is made par- 
ticularly manifest by the study of mind. 
