§ X.] Parasitism. 1 09 



done, and no instance of the kind is to be found in nature. In 

 these cases the parasitism is obligate and indeed strictly obligate. 

 I add the word strictly, because there is a modification of 

 obligate parasitism, in which a parasitic mode of life is necessary 

 for the completion of the entire course of the development, and 

 is often the only one which actually occurs, while at the same time 

 saprophytism may take its place, at least in certain stages of the 

 development. No example of this kind occurs to me at this 

 moment from the animal kingdom, but there are some to be 

 found. Among the Fungi there are a number of species of the 

 genus Cordyceps which inhabit insects, especially caterpillars, 

 and in which this adaptation occurs in a marked manner. The 

 germ-tubes developed from spores on the caterpillar penetrate 

 into the insect, spreading luxuriantly in it, and at length killing 

 it, and after its death they fill the whole of its body with mycelial 

 tissue. From this tissue, if the conditions are favourable for 

 vegetation, large Fungus-bodies are produced, several inches in 

 length, which are the stromata of the Fungus, and produce spores 

 (ascospores). These go through the same course of development, 

 if they also find their way to a suitable living insect. But if this 

 does not happen, the spores have the power of germinating on 

 a dead organic substance, for instance in a nutrient solution, 

 and their germ-tubes may develope there into Fungus-plants. 

 But these plants do not produce the characteristic stromata just 

 mentioned. They form different spores from those produced 

 in the stromata, and these spores may also develope in the 

 saprophytic mode of life ; but if they find their way to the 

 proper insect-host, they can recommence the course of develop- 

 ment which reaches its highest point in the formation of stro- 

 mata as described above. Here then we have parasites 

 which are able to complete a certain portion of the course of 

 their development while living as saprophytes, though without 

 reaching its highest point, namely the formation of stromata ; 

 they may be shortly termed facultative saprophytes. 



Thirdly, there are also facultative parasites. These are 



