934 
Here, again, a wide field for investigation 
presents itself. For example, in man exer- 
cise does not merely cause a muscle to in- 
crease in size ; it occasions besides, as in 
athletes after training, an increase in effi- 
ciency (1. e., in the power and duration of 
contraction), which is greatly out of pro- 
portion to the increase in size. Intermit- 
tent friction or heat or other irritant (e. g., 
chemical) not merely causes the skin to 
thicken, as in corns and callosities ; it ren- 
ders it denser also. 
(that is use) may result in change which 
is wholly qualitative. Thus eyes which, 
when unaccustomed to the task, are ren- 
dered sore by the continued scrutiny of 
small objects (e. g., print, as in the case ofan 
adult learner) may by practice be trained, 
without apparent physical change, to endure 
this proceeding without damage. Most of 
these qualitative changes are best studied 
in connection with mind, but there is one 
series, of vast importance to the higher 
animals, and especially to man, which is 
entirely physical, and to which I may, in 
conclusion of this part of my subject, draw 
attention. I allude to the power which 
Natural Selection has developed in high 
animals of acquiring capabilities of resist- 
ing various poisons, particularly those ‘of- 
fensive and defensive poisons (toxins) 
which are secreted by various plants and 
animals. Thus men acquire greatly in- 
creased powers of ‘tolerating’ nicotine and 
opium, which are towins secreted respectively 
by the tobacco and poppy plants to protect 
them from organisms to which they are 
liable to falla prey. Thus also man may 
acquire the power of‘ tolerating ’ the poisons 
of various species of parasitic micro-organ- 
isms which afflict him and the higher 
animals, and which are the cause of that 
great class of diseases known as zymotic. 
These toxins, also, are defensive products 
by means of which the pathogenic organ- 
isms ward off the attacks of the phagocytes 
SCIENCE. 
Again, stimulation. 
[N. S. Vou. VI. No. 156. 
in the blood of the host by which they are 
liable to be destroyed.* When the phago- 
cytes, through experience of the toxin, ac- 
quire the power of tolerating it they de- 
stroy the microbes whereby acquired immu- 
nity is achieved, as in chicken-pox, measles, 
scarlatina, small-pox, typhoid, ete. With- 
out this power of acquiring immunity 
(temporary or permanent) there could be 
no recovery from such a disease as measles, 
for instance ; and therefore, since, unlike 
malaria, for instance, most of these dis- 
eases pass from one infected person to an- 
other, and are for that reason diseases of 
comparatively dense populations. Without: 
this power of acquiring immunity no dense 
populations could exist. In other words, 
if this power of acquiring immunity had 
not been evolved in man, civilization would 
have been impossible. 
I have said that the power of acquiring 
physical traits does not exist among low 
avimals, or, if it exists, does so in propor- 
tion as they are lowly placed in the scale 
of life, to an extent very small as compared 
to its development among high animals. If 
I am right as to this, low animals (e. g., 
invertebrates) should be incapable or little 
capable of acquiring immunity against 
zymotic disease. I am not aware, how- 
ever, that any observations on the subject 
have been made. 
I dare say that many who read the fore- 
going will be inclined to dispute the facts 
and inferences put forward, and to urge, for 
instance, that I have not established any 
proof, nor even brought forward convincing 
evidence, of the truth of my assertion that. 
low animals are incapable, or less capable 
than high animals, of acquiring physical 
characters. There is, in truth, no literature 
to which I can appeal, for the question is 
*Vide The Present Evolution of Man, pp. 199-32 
(London, Chapman & Hall, 1896), and an ‘ Address 
on Acquired Immunity,’ by the author, Lancet; Sep- 
dember 11, 1897. 
