716 William Patten 



to the Coeleuterates and lower Mollusc8. a graduai increase iu the 

 uumber and complexity of these orgaus is very easily understood, siuee 

 the more there are, the better it is for the animai. This proeess would 

 go on until by a very graduai series of changes they were converted 

 into orgaus capable of perceiving objeets, or fine differeuces in the gra- 

 dation of light. There would be a tendeucy for ali of the organs to be 

 modified in the same direction, until finally they are converted into 

 many structural ey es (as in Pecten and Arca). But under general cir- 

 cumstances, the uumber of eyes necessary for an animai is very small, 

 so that a degeneration would set in by which the organs would be re- 

 dueed in uumber. Such appears to be the case with Pecten and Arca. 

 But in many instances it is supposable that the degeneration would not 

 be rapid, for both functions would be held at once; there would 

 be too many eyes for seeing, but not too many for the absorp- 

 tion of energy. For instance the amouut of energy absorbed would 

 depend upon the most perfect condensation of light upon a given area; 

 as we shall see later, this energy is absorbed in the higher forms, directly, 

 by especially modified nerve fibres. Thus it happens that an organ 

 most perfectly adapted for the condensation and absorption of the 

 greatest amount of energy, is likewise perfectly constructed for the 

 perception of objects. The formation of an inverted image in a visual 

 organ is an incidental result of the most advantageous con- 

 centration of light; but this is just the condition essential for 

 the most perfect heliophagous organ. Therefore we bave an 

 organ that can perform to great advantage two functions at 

 once; this is the essential, and only possible condition, upon 

 which we are able to explain the change of function in organs. 

 However perfect the conditio ns might be for the perception of objects, 

 no true visual organs would be produced, and therefore no perception 

 of objects, until the various Systems of organs were suffici ently deve- 

 loped for the performance of this highly complex proeess. The con- 

 struction of the eye of Charyhdaea, is sufficìently perfect for the per- 

 ception of objects. But ìf we imagine the most perfect Vertebrate eye 

 attached to a Coelenterate, there could never be any perception until 

 other faculties were developed in the highest degree. There is a 

 greater difference between the most simple and most complex Vertebrate 

 retina, than between the most simple Invertebrate one and that of 

 Pecten. In fact, the latter excels in complexity that of the lower fishes, 

 or of certain reptìles; it also possesses an advantage over the retina 

 of ali Vertebrates in the absence of a blind spot. 



