most of them accepted
the above explanation. Mr. T. D. A. Cockerell, however, advanced another
and very ingenious hypothesis, showing that there is certainly one
conceivable way in which natural selection might have produced all the
phenomena of gall-formation by acting directly on the plants
themselves[65]. Subsequently Mr. Cockerell published another paper upon
the subject, stating his views at greater length. The following is the
substance of his theory as there presented:--

[65] _Nature_, vol. xli, p. 344.

Doubtless there were internal plant-feeding larvae before there
were galls: and, indeed, we have geological evidence that boring
insects date very far back indeed. The primitive internal feeders,
then, were miners in the roots, stems, twigs, or leaves, such as
occur very commonly at the present day. These miners are
excessively harmful to plant-life, and form a class of the most
destructive insect-pests known to the farmer: they frequently cause
the death of the whole or part of the plant attacked. Now, we may
suppose that the secretions of certain of these insects caused a
swelling to appear where the larvae lived, and on this excrescence
the larvae fed. It is easy to see that the greater the excrescence,
and the greater the tendency of the larvae to feed upon it, instead
of destroying the vital tissues, the smaller is the amount of harm
to the plant. Now the continued life and vitality of the plant is
beneficial to the larvae, and the larger or more perfect the gall,
the greater the amount of available food. Hence natural selection
will have preserved and accumulated the gall-forming tendencies, as
not only beneficial to the larvae, but as a means whereby the
larvae can feed with least harm to the plant. So far from being
developed for the exclusive benefit of the larvae, it is easy to
see that, allowing a tendency to gall-formation, natural selection
would have developed galls exclusively for the benefit of the
plants, so that they might suffer a minimum of harm from the
unavoidable attacks of insects.

But here it may be questioned--have we proof that internal feeders
tend to form galls? In answer to this I would point out that
gall-formation is a peculiar feature, and cannot be expected to
arise in every group of internal feeders. But I think we can afford