112 Lectures on Bacteria. [^ x. 



also strange exceptions occur within a cycle of preference, some 

 host-species being spared by the parasites without any definable 

 reason. To name only one instance, a Fungus named Phyto- 

 phthora omnivora from its catholicity of taste attacks the most 

 heterogeneous plants, species of Oenothera and other herbs and 

 garden-flowers, Sempervivum, the beech, &c. ; on the other 

 hand it never attacks the potato-plant, which its nearest relative 

 Phytophthora infestans prefers. 



It is at present scarcely possible to give an exact account of 

 the physiological causes of these preferences, but it is at the 

 same time obvious that they depend essentially on chemical and 

 physical qualities and distinctions. 



If then there is a choice between one species and another, 

 there must be a similar choice to a certain extent between indi- 

 viduals of the species, for the differences between the several 

 species are the same in kind, though not in degree, as those 

 between individuals of one and the same species ; the latter are 

 less than the former, and are therefore also less pronounced, 

 being sometimes scarcely or not at all perceivable ; gradations 

 between the different cases which we meet with everywhere are 

 also not wanting here. 



If we describe these phenomena which run through the whole 

 of the long series of parasites in the reverse way, that is to say, 

 not with reference to the parasite but to the host, then we say 

 that the host is differently suited, disposed, or predisposed, ac- 

 cording to the species and individual for the attack of a parasite. 

 We may speak of predisposition on the part of a species, or 

 individual, or in different states, stages of development, or ages 

 of an individual. Of these individual predispositions it may be 

 further specially observed that they must, no less than all others, 

 as a general rule, have their foundation in each case in the 

 chemical, physical, and anatomical constitution. It may be 

 shown for example in the case of certain Fungi of the genera 

 Pythium, Sclerotinia, and others which live in plants, that indi- 

 viduals of the same host-species have unequal susceptibility to 



