474 
Saw-flies, Gall-flies, Ichneumons, 
These set the whole physiological apparatus in motion, and secure the 
insertion of eggs at the right time and in the right place. The number 
of eggs placed is instinctively proportionate to 
the space suitable for oviposition, to the size of 
the fully grown galls, and to the food-supplies 
available for their nutrition. Dryophanta scutellaris 
will only place from one to six eggs on a leaf which 
Neuroterus lenticularis would probably prick a 
hundred times.” 
“ Whatever form the gall takes, the poten¬ 
tialities of the tissue-growth exhibited by it must 
be present at the spot pricked by the fly.” 
“The potentialities of growth being present, they 
are called into activity by the larva, a result advan- 
Fig. 667 —Cynips quercus- tageous to the larva and sometimes described as 
saltatnx, the gall-fly . ir ... . 
which produces the disinterested and self-sacrificing on the part of the 
jumping galls. (Much plant. We have just seen that, so far as the larva 
enlarged.) . g concernec ^ the peculiar structures of the gall 
owe their origin to their success in feeding and defending it; and, so far as 
the plant is concerned, these structures have been evolved in consequence 
of their value in enabling the plant to repair injuries in general, and the 
injuries inflicted by larvae in particular. If John Doe raises a cane to strike 
Richard Roe, and Richard throws up his arms intuitively to parry the stroke, 
the action does not indicate a prophetic arrangement of molecules to frustrate 
John in particular, but an inherited action of defence. The first act of an 
injured plant is to throw out a blastem, and only those larvae survive to hand 
down their art which emerge from an egg so cunningly placed as to excite the 
growth of a nutritive blastem. It is not always possible to keep the besiegers 
from using the waters of the moat, although there is no disinterested thought 
of the besiegers’ wants when the ditches are planned. So in the war-game 
that goes on between insect and plant, natural selection directs the moves 
of both players, but there is nothing generous or altruistic on either side.” 
The exact character of the plant’s abnormal growth has been recently 
studied by several investigators. Cook, an American student, concludes 
from his studies that in the formation of all leaf-galls (except the Cecidomyid 
or dipterous midge-galls) the normal cell-structure of the leaf is first modi¬ 
fied by the formation of a large number of small, compact, irregular-shaped 
cells. The mesophyll is subject to the greatest modification and many small 
fibro-vascular bundles form in this modified mesophyll. Both Adler and 
Sockeu consider that after the first stages of formation the gall becomes an 
independent organism growing upon the host-plant. Cook believes this 
to be true of the Cynipid galls. A surprising conclusion arrived at by Cook 
