Eyes of Molluscs and Arthropods. 715 



consequeutly it required no great effort of the imagination to suppose 

 that tlie black surface absorbed the heat, and produced an impression 

 lipon the animai. But this siipposition was founded upon ignorance, or 

 blindness to the faet that by far the majority of the simplest eyes 

 contain only red pigment, which, as far as we know, has no special 

 power to absorb heat. 



Even if we grant that the ocelli distingiiish the amount of heat, or 

 even light from darkness , neither siipposition woiild fiirnish any ade- 

 quate explanation of the hundreds, and, in some cases, thousands of eyes 

 present in certain Coelenterates and Molluscs. If, according to the exist- 

 ing suppositions, it is difficult to ascribe any function to these numerous 

 ocelli, and therefore any necessity for their existence, it is infinitely 

 more difficult to imagine any sufticient cause for their creation! But 

 I think that it will be generally admitted that all these difficulties are 

 removed, or at least rendered comparatively easy to explain, if we 

 accept the suppositions we have advanced in these pages. 



We have offered some reasons for supposing that (1) animai pig- 

 ment has a physiological activity beneficiai to the animai ; (2) the ac- 

 tivity, and existence of the pigment, depends upon the presence of sun- 

 light; (3) that animai, and plant pigment, or Chlorophyll, represent two 

 phases of the same substance, and have essentially the same function, 

 i. e. the absorption from the sunlight of the energy necessary for the maiu- 

 tainance of vital processes ; (4) that the highly modified plant or ani- 

 mal pigment, or the red pigment which we shall call ommerythrine, 

 has this property to a special degree. According to these suppositions 

 we have no difficulty in ascribing a necessary function to the sim- 

 plest ocelli, neither is there any difficulty in tracing an uninterrupted 

 series of changes from collections of pigment or Chlorophyll granules, 

 to an undoubted ocellus, and these changes necessitate a eorresponding 

 increase of advantages to the animai. We may also follow the devel- 

 opment of the simplest ocellus from a red pigment spot of the plant 

 zoospores, to the most perfect Vertebrate eye, without a break in the 

 functional activity of the organ, or in the advantage gained from it by 

 the animai. We can also trace a complete series of changes 

 from the absorption of light energy by means of pigment, 

 or by nerves, up to the perception of light. We are also enabled 

 to account for the increase in the number of the organs in various 

 groups of animals and their subsequent diminution in numbers. It also 

 furnishes us with the only adequate explanation of the number of highly 

 developed eyes in Pecteti. For instance, in ascending from the Protozoa 



