MESSMATES, MUTUALISTS, AND PARASITES 19 
fly punctures a bud, or leaf, or stem, by means of her sharp 
ovipositor, and lays an egg in the incision. The injury is trifling, 
but sets up irritation, probably caused by some secretion, and the 
result is an abnormal growth. Some of the different galls to be 
seen on oak-leaves are represented in fig. 1074. Other examples 
are furnished by the familiar “oak-apples”, and the “ bedeguars ” 
of rose-bushes. A particular species of gall-fly always selects the 
same sort of plant, and attacks the same region, the resulting gall 
being of definite size, shape, and colour. A remarkable case is 
cited below (see p. 81), where the 
gall benefits the plant on which it is 
found. 
DEFENCES OF PLANTS AND ANIMALS 
AGAINST ONE ANOTHER.—A good deal 
of space has already been devoted 
(vol. ii, p. 275) to the innumerable 
devices by which various animals are 
more or less protected in reference to 
carnivorous forms; but animals are 
also liable to be attacked by plants, 
especially by microscopic but deadly 
bacteria that induce many sorts of 
disease, particularly those of infectious 
or contagious nature. One important 
function of the white or colourless cor- 
puscles which live in lymph or blood : 
appears to be to repel the attacks of Fiz Sica it as 
dangerous “germs” of the sort (see 
vol. iii, p. 4). The principle involved in vaccination or inocu- 
lation is related to the fact that animals which have been pur- 
posely subjected to the influence of a disease-germ that has been 
weakened by artificial methods (or to the action of a related but 
less dangerous kind of germ), are thereby rendered able to resist 
more or less successfully the onslaughts of the same sort of germ 
in its more virulent form. Another important application of pre- 
ventive (and curative) medicine has resulted from the discovery 
that some animals are protected from particular disease-germs by 
means of complex substances (defensive proteids or anti-toxins) 
contained in their blood. The best-known example is afforded 
by diphtheria, which can be warded off, or combated if con- 
