61 
Naxepa’s opinion however, this manner of distribution can not - 
occur very regularly, since if it did it would be hard to ac- 
count for the fact so often observed, that of two trees growing 
in close proximity, one is and the other is not infected. Even 
for Krakatau the possibility of gall-mites having been conveyed 
there by other animals cannot be entirely excluded, though it 
seems to me improbable that this manner of distribution should 
take place with any regularity over such long distances, consi- 
dering that gall-mites do not possess strong limbs to fasten 
themselves to other creatures. Moreover it will be a very difficult 
thing actually to observe such a transport. 
There remains then the distribution of the insects by the 
wind. They are indeed small and light enough to be possibly 
earried by winds over a considerable distance; besides, the plant- 
leaves with galls may drop into the water and be carried along 
in that way. Now it is not known at all whether mite-galls 
may float for any length of time in water, and specially seawater 
without their inmates perishing. Until this point has been in- 
vestigated, it will be necessary to adhere to the theory of the 
distribution of the insects by the wind. ; 
In Késrers ') work on galls an entire chapter is devoted to 
the discussion of the spread of gall-producers. Kiser discusses 
the spread of eel-worms (but no such gall-producers have so 
far been found on Krakatau), of gall-mites and of gall-gnats. 
In discussing the spreading of the gall-mites Kuster arrives 
at the same conclusions as Napa, but in addition he draws 
the following inferences: A. Many gall-producers extend their 
range or area more slowly than the plants they live on. 
(This appears to me far from certain and in any case it is not 
proved). B. Where the confines of the gall-producers and the 
host plants coincide, the conclusion may be drawn that the limits ~ 
of the plant-area have remained unchanged for a long time. 
(This follows from the first thesis and is, in my opinion, equally 
= _ Open to doubt). C. If no galls are to be found on the confines — 
a of a plants area, it may be Haprred | that the plant is still 
— Oe ere ‘Die gallen der Pflanzen. Leipzig 1914, p. 36 
