ON BIOLOGICAL ANALOGIES. 279 



did not fail to do, though the fungus is at present not known as 

 indigenous to this country. 



" The subject has, however, been recently called more forcibly to 

 our notice from some remarkable phenomena which have occurred in 

 a garden near London, where all the plants of Pyracantha arising 

 from particular seed are more or less completely destroyed by the 

 same species of Cladosporium which has injured the pear trees. The 

 plants were raised from Russian seeds, about four or five years 

 since. They first appeared to be blighted last year, but inconsider- 

 ably. This year they are nearly all killed. The old shoots are 

 black with the spores, the leaves crumpled and withered ; and the 

 late shoots and leaves now forming are about to break out as their 

 predecessors. The seedling bushes are in heavy land well drained, 

 but the disease is confined to them ; old bushes of Pyracantha in 

 the same places are perfectly clear."* 



It may be asked if any of the diseases of plants are infectious in 

 the same manner as small-pox or foot-and-mouth disease. Un- 

 doubtedly this may be answered in the affirmative, as the potato 

 disease, the hollyhock disease, the coffee leaf disease, and the vine 

 mildew would prove. They may be disseminated by inoculation, 

 and there is no reason why some of them may not be mitigated by 

 some process analogous to vaccination. The larch disease extends 

 over large tracts, but it cannot be placed in the same category. 

 Willkomm endeavoured to show that the larch disease was produced 

 by a Peziza, but I think that he has failed to establish it, both by 

 fact and analogy, for it appears to be traceable rather to resinosis, 

 of which the fungus is only an adjunct, growing upon the diseased 

 tissue, and that resinosis, like gummosis, is not infectious. 



It cannot be a trifling circumstance that the invasion of our island 

 by all the principal epidemics affecting plants, have coincided re- 

 markably with the epidemics affecting animals. The hollyhock 

 disease (Puccinia malvacearwn) first known in South America, 

 travelled to Australia into Europe, and at length made its appear- 

 ance on the coast of England, on the continental side, travelling 

 gradually onwards until it covered the entire country, in successive 

 waves, just as cholera or cattle disease would do if left to them- 

 selves, and as the potato disease had done before. | It may be only 



* M. J. B., " Gard. Chroii.," Oct. 28, 1848, p. 716. 



t The potato disease appears to have been observed in Belgium in 1842, 

 according to Morren, and about the same time it was stated to have been 



