instances be quickly followed by a more gross mechanical contact, the rudimentary visual impression is, as Spencer says, a kind of "anticipatory touch." From this simple beginning, in which bodies only slightly separated from the impressible foci excite certain general or only vaguely specialized impressions corresponding to light and shade therein, the organs of sight and their impressibility gradually become more and more elaborate. To rudimentary aggregations of pigment transparent media are added, which condense the light on these impressible patches, and these media in other organisms are sufficiently like a lens to be adequate to form a definite image of an external body on the layer of pigment, which, on its other side, is in contact with a nerve-expansion communicating with a contiguous ganglion. Numerous simple structures of this kind may exist apart from one another, as in many bivalve mollusks, or they may be far more numerous and closely aggregated so as to form such compound eyes as are met with in crustaceans and in insects. Or individual ocelli may be perfected, as in spiders, or lower crustacea, though most notably of all among the cuttle-fish tribe in which two movable eyes are met with, whose organization is just as perfect as that of the eyes of fishes.
The difference in degree and range of sensitiveness existing between the simple "eye-specks" of some of the lower worms and the elaborate organs existing in the highest insects and mollusks is enormous. The range and keenness of vision become progressively extended, so that creatures with more perfect eyes are capable of receiving and appreciating impressions from objects more and more distant, and the various actions which become established in response to impressions habitually made upon such sensitive surfaces increase enormously in number, variety, and complexity. The relation existing between the keenness of the sense of sight and the powers of locomotion of insects has long been recognized by naturalists. Prof. Owen, for instance, thus alludes to it: "The high degree in which the power of discerning distant objects is enjoyed by the flying insects corresponds with their great power of traversing space. The few exceptional cases of blind insects are all apterous, and often peculiar to the female sex, as in the glow-worm, cochineal-insect, and parasitic stylops."
The various actions of insects and of invertebrate animals generally are, however, found to be easily capable of classification. They are, in the main, subservient to the pursuit and capture of prey, to the avoidance of enemies, to the union of the sexes, or to the care of their young. To such ends are their various motions, whether occasional or habitual, more or less directly related. Nothing is here said, however, as to the extent to which such ends are realized by the animals themselves.
In vision, as I have said, we have to do with a refinement of the sense of touch, whereby the animal becomes sensible of impressions
