E. J. BUTLER 7 



records in the case of rusts in which there are two hosts, where one 

 appears to have become infected from the other through moderate dis- 

 tances. Coleosporium Euphrasice seems to reach the islands of? the north- 

 west German coast from the mainland^ and other cases are recorded 

 of the occurrence of one stage of these fungi from 5 to 8 miles from the 

 plant bearing the alternate stage. The general results of the investigations 

 seem to show that the distance to which spores may be carried in the air has 

 often been exaggerated in the past, and that it is much less than might be 

 expected.^ Even such small bodies fall gradually when in the air and it has 

 been found^ that the spores of some of the higher fungi fall at the average 

 rate of 0*5 — 5 mm. per second in still air, and so must come to earth before 

 very long. 



On the other liand, it is well established that dust particles (which are of 

 the same order of size as spores) can be carried to great distances under certain 

 conditions. The dust from the volcanic eruption of Krakatao in 1883 travelled 

 thousands of miles, that from Vesuvius and Etna has reachefl Constantinople, 

 Norway has been strewn with volcanic dust from Iceland. In these cases the 

 dust has been probably thrown up to a great height by the eruption and its 

 lateral spread may be accounted for by this. But dust has also been carried 

 a long way when blown from the ground by storms. From North Africa it 

 is said to have reached a considerable part of southern Europe ; while the 

 wind kno-uni as the Harmattan, which takes its rise in the Sahara, blows clouds 

 of dust far out into the Atlantic in a south-westerly direction from the Sene- 

 gambia and Guinea coast. This dust has often fallen on vessels several 

 hundred, and even more than a thousand, miles out to sea, and Darwin col- 

 lected particles of stone, over a thousandth of an inch square, 300 miles from 



' This and several other interesting cases are quoted by Klebahn (" Wirtswechsehiden 

 Rostpilze " 1904.) 



2 I believe a somewhat similar conclusion has been reached by plant breeders with regard 

 to the distance to which pollen grains may be carried by the wind ; and the long controversv 

 with regard to air-borne infection in such human parasitic diseases as scarlatina and cholera has 

 ended in the same way. Indeed the whole of our modern sanitary regulations deahng with 

 quarantine and the restriction or isolation of infectious diseases, are based on the view that it 

 is the hving carrier and objects, such as clothing, wliich have come into contact with infected 

 persons, that constitute the danger. Even the long-lived and minute organism of scarlatina is 

 not beheved to be carried in the air to more than a very limited distance and did not reach many 

 parts of the world until long after it had been known in Europe. As would be expected tho 

 evidence regarding the dissemination of the seeds of higher plants is to the same effect and oven 

 the small spores of ferns have not been of advantage to this group in enabling it to colonise 

 distant areas more readily than the flowering plants (H. Christ, " Die Geographie der Fame," 

 1910). 



8 BuUer, A. H. R. " Researches on Fungi," 1909, Chaps. XV and XVI. 



