CHAPTER VII.—PHENOMENA OF VEGETATION. —PARASITES. 359 
are parasitic on plants; of parasites on animals Laboulbenia Baeri is found only on 
house-flies. Very many kinds thrive on a larger or smaller circle of nearly allied 
species which serve them as hosts; among these are many Uredineae, Ustilagineae, 
and Peronosporeae, Epichloe typhina which lives on the Gramineae and Claviceps 
purpurea; Cordyceps militaris grows on insects of various orders, especially 
Lepidoptera ; C. cinerea, as far as is known, only on species of Carabus, other kinds 
only on wasps, and so on. Some kinds make specific exceptions within the immediate 
circle. of affinity of their host, or they occasionally travel beyond that circle; I 
succeeded for instance in transferring Puccinia suaveolens, which usually lives on 
Cirsium arvense and Centaurea Cyanus, to Taraxacum but not to Tragopogon; 
Phytophthora infestans, which is usually confined to the Solanaceae, is found excep-- 
tionally on the Scrophularineae (Anthocercis viscosa, Schizanthus Grahami), Perono- 
spora parasitica of the Cruciferae on Reseda luteola. 
This exceptional power of accommodation forms the passage to the third category, 
that is, to parasites which attack plants and animals of very different cycles of affinity 
either without any distinction whatever or with a preference for certain species. 
Examples of parasites of this kind living on plants are species of Erysiphe, as 
E. guttata which lives on the leaves of Corylus, Carpinus, Fagus, Betula, Fraxinus, and 
Crataegus, Phytophthora omnivora which attacks Fagus, Sempervivum, the Oenothereae 
and other plants, but not Solanum tuberosum’, and Sclerotinia Sclerotiorum, which 
can penetrate as a parasite into the most diverse juicy parts of plants. Of cases of 
parasites on warm-blooded animals may be mentioned Lichtheim’s Mucor rhizopodi- 
formis, one of the pathogenous Moulds which will not develope on the dog, but grows 
vigorously in the rabbit; Aspergillus fumigatus attacks both of these animals; no 
others have been tried. For further examples the reader is referred to descriptive and 
pathological treatises. 
These facts and gradations would lead us to expect that there must also be 
differences in the aggressive behaviour of a parasite to the different varieties and in- 
dividuals of a host; or, to express the matter in the converse way, in the predisposction 
of the individuals for the attacks of the parasite. In this direction also there are all 
possible gradations. On the one hand there are parasites which, as far as we know, 
show no preferences of the kind, for instance all the strictly parasitic species of the 
genus Peronospora and of the group of the Uredineae in which this point has been 
examined. The other extreme is represented by the Saprolegnieae for example, 
which attack fishes, and by the Sclerotinieae and Pythieae, which as facultative 
parasites attack Phanerogams. These will be discussed at greater length in a suc- 
ceeding page (see p. 380). The physiological reason for these predispositions cannot 
in most cases be exactly stated; but it may be said in general terms to lie in the 
material composition of the host, and therefore to be indirectly dependent on the 
nature of its food. In the case of the Pythieae, for example, it is easy to see that the 
host displays degrees of susceptibility or power of resistance in presence of the parasite 
proportioned to the amount of water which it contains. It must on the whole be 

-2 Bot. Ztg. 1881, p. 595. 
2 On the disposition of plants see Sorauer, Landw. Versuchsstationen, XXV (1880), p. 327, and 
the discussion in Bot. Ztg. 1882, pp. 711 and 818. The questions of disposition and immunity in the 
case of diseases of animals caused by parasites are fully discussed in medical literature. 
