16 HETERGCISM 
adapt itself to new conditions and thus tends to perpetuate the 
race, while the change of host, which is equivalent to a change 
of diet, may very possibly tend to an increase of vigour in the 
individual. 
The former statement, however, must not be taken to refer 
to all cases of hetercecism, since there are instances, e.g. in 
species of Coleosporium, which cannot be explained on this 
ground. 
The reason why P. Caricis has been taken as the typical 
Uredine, instead of the usual P. graminis, is that the ecidium 
of the latter is now very rarely found in this country and is 
therefore not available for demonstration, while that of P. Caricis 
is common in all suitable localities. Even if not existing in 
any place, it can be readily introduced if the three prerequisites 
are at hand:—a pond bordered by Carex paludosa and by 
Urtica dioica, and also a quantity of the leaves of Carex 
infested by the parasite. The Nettle and the Sedge are not 
injured appreciably by the disease, nor would it be of much 
consequence if they were. To introduce the fungus into the 
new locality, it is only necessary to obtain a bundle of the 
required leaves (say about 500) from some place where the 
Pucciniu exists, in January or February, and lay them on 
the ground where a patch of nettles is known to occur. The 
latter will be seen in spring to be beautified by the ecidium, 
and in the summer the disease will spread to the surrounding 
sedges (see Grove, Journ. Bot. 1913, p. 42). 
4 
