That the former ia done in compliance with a general law can hard 
doubted, when we consider how effectually and persistently this singling c 
weak specimens is carried out, for the evident purpose of their ultimate de, 
tion and replacement of healthier individuals. 
As a case in point, I offer in this paper an account specifying the long 
tmued attacks of Cecidomyia m<vrginemtorquens, Bremi, on a plant of Salix vimi 
My diary records that — 
1866, August 3.-I found the marginal rolls of this Cecidomyia, col. 
alternatively red and yellow, in great numbers on the leaves of a straggUng 
of Sahx vzminalis, in the hedge-row of a garden at Penge. The willow 1 
ragged appearance, branches have been violently torn off, the bark remoy, 
parts and the leading shoot is destroyed. The bush looks as if it had suf 
repeatedly by the ruthless passage of man and beast, it being situated ir 
weakest part of the hedge. 
1866, August 17.— Still abundance of tenanted rolls, 
1867, May 22._Same willow in the hedge nearly trodden down by ci 
Fresh roUs on many leaves of the intact shoots. 
1868, June 5.-Plenty of rolls on the willow in the hedge on the few brat 
remammg ; many shoots are clipped off. 
1868, June 7.-Bred from such rolls both sexes of Cecidomyia margi. 
torquens, Bremi, in numbers. 
1868, September 27.-Noticed chains of fresh roUs on the same bus 
abundance. 
1869, May 23.-Found first leaf this year bordered with several rolls on , 
1869, July 25.-Eolls in excessive numbers j majority of leaves thus infos 
Scarcely any young shoots of the year growing healthily. 
_ During all these years I have diligently searched the other bushes of 
willow m the same hedge for these galls, but can find none. In one of the ne 
bourmg gardens there grows a fine tall tree of the same Salix ; thinkinc. 
the straggling bush in the hedge might annually receive its settlers from the: 
have repeatedly, and at various times of the year, examined it, but always wii 
negative result. Hence I am driven to the conclusion, that having accident 
been peopled with this gall-midge from a distance, this straggling bush, altho 
plenty more grow near, is annually fixed upon by the laying females as the . 
smtable for their purpose, and will eventually succumb to the rava-es of tl 
progeny as one brood of larvae succeeds the other, at intervals of about four we 
from May to October, the last brood hybernating in their cocoons in the fa 
leaves till spring. 
C. saUcis.-From galls, encompassing some twigs of the preceding ye, 
growth of a plant of Salix cinerea, collected near Penge, on the 11th Octol 
1868, I bred a few specimens of this midge on the 26th June 1869 In t 
particular instance each gall harboured but one larva, and all I met with wi 
full fed or nearly so. They remained in their larva state imtil the middle 
side. 
