GANGLIA OF SPECIAL SENSE :—INSTINCTIVE ACTIONS. 381 
the brain in the Invertebrated animals; and in Fishes they 
bear a very large proportion to the other parts, their relative 
size gradually diminishing as we ascend the scale towards 
Man. Now when we study the actions of these lower tribes 
of animals, we find that those which evidently depend upon 
sensation , especially the sense of sight, are very far from 
being of the same spontaneous or voluntary character as those 
which we perform. We judge of this by their unvarying 
nature,—the different individuals of the same species execut¬ 
ing precisely the same movements, when the circumstances 
are the same,—and this evidently without any choice, or 
intention to fulfil a given purpose, but in direct respondence 
to an internal impulse. Of this we cannot have a more 
remarkable example than is to be found in the operations 
of Bees, Wasps, and other social Insects; which construct 
habitations for themselves upon plans which the most enlight¬ 
ened human intelligence could not surpass; yet which do so 
without hesitation, confusion, or interruption,—the different 
individuals of a community all labouring effectively for one 
common purpose, because their impulses are the same (Chap¬ 
ter XIV.) 
476. In higher animals we may often notice the effect of 
similar promptings, by which the various species are guided 
in their choice of food, in the construction of their habitations, 
in their migrations, &c. : but these are frequently modified 
to a certain degree by the intelligence which they possess. 
The closure of the pupil when the eye is exposed to a strong 
light, and its dilatation when the light diminishes (§ 534), 
afford a very marked example of this “ consensual” class of 
movements, which differ from the simply-reflex in requiring the 
stimulus of sensations, but which are, like them, quite indepen¬ 
dent both of the reason and of the will. A still more striking 
illustration, however, is furnished by the mode in which a 
little Fish, termed the Chcetodon rostratus, obtains its food. 
Its mouth is prolonged into a kind of beak or snout, through 
which it shoots drops of liquid at insects that may be hover¬ 
ing near the surface of the water, and rarely fails in bringing 
them down. Now, according to the laws of Optics, the insect, 
being above the water whilst the eye of the fish is beneath 
it, is not seen by it in its proper place ; since the rays do not 
pass from the insect to the fish’s eye in a straight line (§ 528), 
