I30 NATURAL SCIENCE. Feb., 



developed, through what was vaguely called " over-cultivation," an 

 inherent tendency to disease, just as we hear a great deal about the 

 " diseases of civilisation," and so on. There may be something in 

 this, but no one has ever proved that there is, and, meantime, one must 

 be content with the apparently sufficient explanation that the cultiva- 

 tion of crops in close contiguity gives the parasitic organisms oppor- 

 tunities of spreading superior to those commonly found in nature 

 (though not so much so as might readily be supposed), and also 

 affords the spectator a better opportunity of witnessing the effects, 

 and attracting his attention, from the fact that the diseased plants 

 were valuable to him, much more than wild plants, for example. 

 One cannot resist suspecting that some of the so-called "diseases of 

 civilisation " are in the same boat. We know that such as are 

 infectious are often infinitely worse among uncivilised communities, 

 among whom they possibly originated, and such as are not infectious 

 are sometimes, at least, imaginary ailments, or ascribed to civilisa- 

 tion on wholly inadequate grounds. But these things are leading me 

 away. We certainly do not know that there is an " inherent tendency 

 to disease produced by over-cultivation," and we do not even know 

 if there be such a thing as an inherent tendency to disease at all. 

 The fact that one species is taken by a parasite and another left, 

 that one variety even may escape wholly or partially while others 

 suffer, shows us that there may be something of the kind ; but, on the 

 other hand, the tendency is better described as on the part of the 

 parasite ; the predisposition may be in the environment of the host, 

 and, lastly, need not be a sickly or enfeebled condition of the host, 

 for that is generally what is meant. 



It may not be too tedious if I venture to recall that these fungi 

 and bacteria all gain their living as parasites producing diseases in 

 living bodies of plants and animals, or as saprophytes feeding on 

 their decaying bodies or dead organic substances. Between the strict 

 parasites and strict saprophytes there are, however, intermediate forms, 

 such as parasites that are /ac/zZ/rtf/w saprophytes and saprophytes that 

 are facultative parasites. Finally, we have in the lichen-forming fungi, 

 for example, cases of commensalism in which a lasting nutritive part- 

 nership is struck with the host instead of a one-sided arrangement, 

 as in the other cases. It will be seen, therefore, that there is a 

 considerable range of nutritive adaptations among these organisms — 

 a certain elasticity of accommodation. Among the parasites, which 

 principally concern us, we have such as are facultative saprophytes 

 and the converse, and we have others that are almost omnivorous 

 parasites, attacking plants of diverse groups ; others confined to an 

 allied group of species ; others to a single species. Among them 

 there are noteworthy instances of parasites that attack hosts outside 

 the group mostly affected, and others again that exempt particular 

 species and even varieties within the group of hosts. Consideration 

 of such instances rouses curiosity as to the exact cause of immunity, 



