Lvidences of Theory of Natural Selection. 293 
On the whole, it seems most probable that the fluid 
is of the nature of an excretion, serving to carry off 
waste products. Such, at all events, was the opinion 
at which Darwin himself arrived, as a result of ob- 
serving the facts anew, and in relation to his theory. 
The other instance to which I have alluded as 
seeming at first sight likely to answer Darwin’s 
challenge is the formation of vegetable galls. The 
great number and variety of galls agree in presenting 
a more or less claborate structure, which is not only 
foreign to any of the uses of plant-life, but singularly 
and specially adapted to those of the insect-life which 
they shelter. Yet they are produced by a growth of 
the plant itself, when suitably stimulated by the 
insects’ inoculation—or, according to recent observa- 
tions, by emanations from the bodies of the larve 
which develop from the eggs deposited in the plant 
by the insect. Now, without question, this is a most 
remarkable fact ; and if there were many more of the 
like kind to be met with in organic nature, we might 
seriously consider whether the formation of galls should 
not be held to make against the ubiquitous agency of 
natural selection. But inasmuch as the formation of 
galls stands out as an exception to the otherwise 
universal rule of every species for itself, and for itself 
alone, we are justified in regarding this one apparcnt 
exception with extreme suspicion. Indeed I think 
we are justified in regarding the peculiar pathological 
effect produced in the plant by the secretions of the 
insect as having been in the first instance accidentally 
beneficial to the insects. Thus, if any other effect 
than that of a growing tumour had been produced in 
