ORGANISMS AND THEIR MEDIA. 219 



instances be quickly followed by a more gross mecbanical contact, tbe 

 rudimentary visual impression is, as Spencer says, a kind of " antici- 

 patory 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 im- 

 pressible 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. Nu- 

 merous 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 be- 

 tween the simple " eye-specks " of some of the lower worms and the 

 elaborate organs existing in the highest insects and mollusks is enor- 

 mous. The range and keenness of vision become progressively ex- 

 tended, so that creatures with more perfect eyes are capable of receiv- 

 ing and appreciating impressions from objects more and more distant, 

 and the various actions which become established in response to im- 

 pressions habitually made upon such sensitive surfaces increase enor- 

 mously in number, variety, and complexity. The relation existing 

 between the keenness of the sense of sight and the powers of locomo- 

 tion 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 corre- 

 sponds with their great power of traversing space. The few excep- 

 tional 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 gener- 

 ally 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 occa- 

 sional or habitual, more or less directly related. Nothing is here said, 

 however, as to the extent to which such ends are realized by the ani- 

 mals 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 



