MESSMATES, MUTUALISTS, AND PARASITES 



79 



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 

 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- 



Fig. 1074. Various Insect-Galls on Leaf 

 of Oak (Qiiercus] 



