§ X.] Parasitism. 1 1 1 



there is every possible gradation again between these extremes. 

 Lastly, there are cases in which a parasite lives in a host with- 

 out either injuring or sensibly profiting by it, at most deriving 

 its food from the refuse of the metabolism of the host. In 

 extreme cases of this kind, which obviously lie on the border- 

 line of the phenomena of true parasitism, we use the expression 

 lodger-parasites. 



Further, there is a fact more or less known to the experience 

 of every one, which holds good of all the categories of parasites 

 distinguishable according to the points of view here indicated, 

 namely that a parasitic species may make choice as we should 

 say between the hosts which it occupies, that is, attacks one host 

 and thrives perfectly well in or on it, while it either refuses 

 others altogether or at least grows less vigorously in them. In 

 these respects also there is every conceivable gradation. First, 

 as regards the choice of the host-species by a parasitic species ; 

 one extreme is marked by the narrowest one-sidedness. For 

 instance, a strictly obligate and very well marked parasitic 

 Fungus, Laboulbenia Muscae, mentioned on page 39, grows 

 exclusively on the house-fly and on no other insect, at least 

 according to our present investigations. Other Fungi and 

 other parasites besides the Fungi are not so one-sided in their 

 choice, since they attack a larger number of host-species, but 

 those only as a rule which belong to a narrow cycle of affinity, 

 a genus, family, &c. Thus, for example, some of the species of 

 Cordyceps mentioned above, grow in the larvae of a great 

 variety of butterflies and other insects. But it sometimes 

 happens that single host-species within such a cycle of affinity 

 remain excluded from the choice for reasons of which we are 

 ignorant. Lastly, we are acquainted with obligate and faculta- 

 tive parasites which are able to complete their development 

 equally well in hosts of the most different cycles of affinity. 

 I need only mention Trichina spiralis once more, which thrives 

 well in rodents, swine, men, and other animals. Examples in 

 plenty may also be drawn from the Fungi ; but among them 



