6 DISSEMINATION OF PARASITIC PUNGil 



infected Hudson River Valley.^ So far as the spread from east to west is 

 concerned, birds can, therefore, have played little part. 



The diseases liable to be carried by birds would naturally be chiefly those 

 affecting fiuit and forest trees and producing spores on the upper parts 

 of the tree. 



Outside birds, the only other animals that might perhaps help to dissem- 

 inate fui^gi over large distaaces are insects. In some cases these have a 

 power of dispersal probably unequalled even amongst biids.^ That they 

 should carry spores would seem to be a most natural assumption. Possibly 

 they have done so in many cases in the past, and there may now only be a 

 residue of cryptogamic parasites in each land area which, for some unknown 

 reason, is unsuitad to dispersal by birds and insects. It is only within com- 

 paratively recent times that any record has been kept of the appearance of 

 new fungal diseases in certain parts of the world, and it is quite possible that 

 all or nearly all of these belong to this residue. Since, however, the object of 

 the present paper is primarily a discusf^ion of the msans by which parasites 

 hitherto confined to one locality are, at the present time, able to roach areas 

 separated by the ocean or by large tracts with unsuitable host plants, and 

 since there is no evidence that birds or insects are now a considerable agency 

 in effecting this spread, what may have happened in the distant past is not of 

 direct consequence. 



A second means of discontinuous spread is through the air. Though it 

 is somewhat difficult to understand why this should not suffice sooner or later 

 to spread most diseases over those parts of the world with suitable climates, 

 yet such evidence as is available points altogether in the opposite direction. 



The bulk of the evidence is circumstantial. Diiect experiments are few 

 and do not seem to have been framed so as to ascertain how far spores might 

 be carried by the wind in storms or when the spores of, say, oat smut are sent 

 into the air in clouds during threshing. Concordant results have been obtained 

 in Germany and Kussia that " spore traps " remained free from smut spores 

 beyond 250 yards from heavily infected fields, though at Pullman, in the 

 United States, the spores may fall copiously at a distance of a quarter 

 of a mile. The area around barberry bushes through which black rust of 

 wheat (which spends part of its life on the barberry) can be disseminated 

 to the wheat is also said to be remarkably low. There are, however, various 



1 Cornell University Agric. Expzr. Slat. Bull, No. 347, 1914, p. 546. 



* Tutt, J. W. " The Migration and Dispersal of Insects," London, 1902. 



