The canon of "Natura non facit saltum" applies with almost
equal force to instincts as to bodily organs. Changes of instinct may
sometimes be facilitated by the same species having different
instincts at different periods of life, or at different seasons of the
year, or when placed under different circumstances, etc.; in which
case either one or the other instinct might be preserved by natural
selection. And such instances of diversity of instinct in the same
species can be shown to occur in nature.
Again as in the case of corporeal structure, and conformably with my
theory, the instinct of each species is good for itself, but has
never, as far as we can judge, been produced for the exclusive good of
others. One of the strongest instances of an animal apparently
performing an action for the sole good of another, with which I am
acquainted, is that of aphides voluntarily yielding their sweet
excretion to ants: that they do so voluntarily, the following facts
show. I removed all the ants from a group of about a dozen aphides on
a dock-plant, and prevented their attendance during several hours.
After this interval, I felt sure that the aphides would want to
excrete. I watched them for some time through a lens, but not one
excreted; I then tickled and stroked them with a hair in the same
manner, as well as I could, as the ants do with their antennae; but
not one excreted. Afterwards I allowed an ant to visit them, and it
immediately seemed, by its eager way of running about, to be well
aware what a rich flock it had discovered; it then began to play with
its antennae on the abdomen first of one aphis and then of another;
and each aphis, as soon as it felt the antennae, immediately lifted up
its abdomen and excreted a limpid drop of sweet juice, which was
eagerly devoured by the ant.
Pages:
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261