E. J. BUTLER 5 



later be able to efiect coutiuuous spread by one or other of the methods 

 discussed above ; for the most part they have ah'eady done so, where not 

 restricted by deficient powers of acclimatisation, and though migrating birds 

 may have enabled them to pass natural barriers such as the Alps or the Channel 

 more rapidly, the present distribution would picbably be nmcli the same even 

 if birds did not exist. The parasites of plants long established in southern 

 and northern Europe are, except in the case of those recently introduced, 

 probably as much identical as they are ever likely to be. Where they differ, 

 the explanation is to be sought in the lesser adaptability to differences in 

 climate frequently possessed by the parasites as compared with their (culti- 

 vated) host plants. 1 In general it may be said that England, for instance, 

 is little liable to receive new pests from southern Europe except when they 

 have been racently introduced into the latter area. The danger, in England, 

 is primarily from more distant regions where the climate is not too dissimilar, 

 such as North America and to a considerable extent also temperate eastern 

 Asia, and birds or other animals cannot greatly increase this danger. 



There is probably an exception in the case of recently introduced pests. 

 Certain classes of these might be rapidly spread within a large part of Europe 

 by birds. Nothing can reduce this danger but we may console ourselves by 

 the reflection that continuous spread through the same areas would almost 

 certainly occur in any case in the long run, and that the remedy lies in keeping 

 such pests out of Europe altogether, by international co-operation. 



Exact investigations of the extent to which birds can actually dissemi- 

 nate fungi seem to be very scanty. In the eastern United States some were 

 examined a short time ago.2 They were shot, and the beaks, legs, etc., 

 scrubbed, the washings being then used to determine the number of spores. 

 On two woodpeckers, 757,074 and 624, 341 spores, respectively, were found and 

 a high proportion of these were the spores of the chestnut bark disease, which 

 is at present exterminating the chestnut in this area. Some of the birds shot 

 were on their northward migration and could no doubt have carried this disease. 

 Yet it is remarkable that though chestnut blight has been in the ireighbourhood 

 of New York since 1904, it had not, up to 1913, been able to cross a 

 chestnut-free belt in the Catskill mountains, some 30 to 40 miles broad, 

 running roughly north and south, parallel to and just west of the heavily 



' Nothing else will explain the absence of Phytophthora injestam in potatoes in the warmer 

 and drier parts of the United States and in the centre and west of India, or of wheat bunt and 

 maizo smut in all but the most temperate parts of India. 



- Jou'ji. Agric. Research, II, 1914, p. 405. 



