BRITISH OAK-GALLS. 
391 
Finally, it was suggested that, instead of considering "physio- 
logical selection " as a new factor in the evolution of species, it 
should he ranked as one of that class of phenomena, called by 
Darwin "variation," and that the law of variation should he sub- 
divided into the following groups : 
A. Variation caused hy isolation and migration. 
B. Variation caused hy climate. 
C. Reproductive variation. 
D. Correlated variation. 
BRITISH OAK-GALLS. 
SYNOPSIS OF LECTURE BY G. C. BIGNELL, F.E.S. 
(Read December 23rd, 1887.) 
Mr. Bignell remarked that it was very interesting to observe the 
different forms of galls. The most extraordinary thing in connec- 
tion with them is, that eighteen species were known to produce two 
distinct forms — one generation producing males and females, and 
the next females only. Not only was there a difference between the 
gall-flies, but also a very great difference between the galls, so much 
so that the two species were separated and placed into two genera. 
These galls were drawn — highly magnified, to illustrate the lecture. 
One familiar species was that of the common oak-apple, perhaps 
better known as King Charles's apple (Spatheg aster terminalis). 
This gall was produced at or near the end of a branch. The flies 
from these galls are of both sexes ; the female deposits her eggs on 
the rootlets of the oak, and the gall formed is very much like a 
small potatoe ; from this gall females only (Biorhiza apterd) are 
produced, and, strange to say, are perfectly wingless, and by a 
general observer would be taken for spiders. In conclusion Mr. 
Bignell summarised the oak-galls known in England, and stated 
that they were to be found on the root, bark, bud, leaf, and catkin. 
The following list of the British oak-galls is arranged in a 
tabulated form, to show the bisexual and unisexual forms — the 
unisexual form is opposite the bisexual form, 
VOL. IX. 2 E 
