204 Darwin, and after Darwin. 
the first instance, or if the needs of the insect progeny 
had not been such as to have derived profit from 
being enclosed in such a tumour, then, of course, the 
inoculating instinct of these animals could not have 
been developed by natural selection. But. given these 
two conditions, and it appears to me there is nothing 
very much more remarkable about an accidental 
correlation between the effects of a parasitic larva on 
a plant and the needs of that parasite, than there is 
between the similarly accidental correlation between 
a hydated parasite and the nutrition furnished to it by 
the tissues of a warm-blooded animal. Doubtlcss the 
case of galls is somewhat more remarkable, inasmuch 
as the morbid growth of the plant has more concern 
in the correlation—being, in many instances, a more 
specialized structure on the part of a host than occurs 
anywhere else, cither in the animal or vegetable world. 
But here I may suggest that although natural selection 
cannot have acted upon the plant directly, so as to have 
produced galls ever better and better adapted to the 
necds of the insect, it may have so acted upon the 
plants indirectly ¢hrough the tnsects. For it may very 
well have been that natural selection would ever 
tend to prescrve those individual insects, the quality 
of whose emanations tended to produce the form of galls 
best suited to nourish the insect progeny; and thus 
the character of these pathological growths may have 
become ever better and bctter adapted to the needs 
of the insects. Lastly, looking to the enormous 
number of relations and inter-relations between all 
organic specics, it is scarcely to be wondered at that 
even so extraordinary an instance of correlation as 
this should have arisen thus by accident, and then 
