78 INTRODUCTION TO ZOOLOGY. 



transition formation, we conclude that they could not have 

 been that imaginary, turbid, and compound chaotic fluid, from 

 the precipitates of which some geologists have supposed the 

 materials of the surface of the earth to be derived ; because 

 the structure of the eyes of these animals is such, that any 

 kind of fluid in which they could have been sufficient [for 

 vision] at the bottom, must have been pure and transparent 

 enough to allow the passage of light to organs of vision, the 

 nature of which is so fully disclosed by the state of perfection 

 in which they are preserved. With regard to the atmosphere, 

 also, we infer that, had it differed materially from its actual 

 condition, it might so far have affected the rays of light, that 

 a corresponding difference from the eyes of existing Crustaceans 

 would have been found in the organs on which the impressions 

 of such rays were then received." 



" Eegarding light itself, also, we learn from the resemblance 

 of these most ancient organizations to existing eyes, that the 

 mutual relations of light to the eye, and of the eye to light, 

 were the same at the time when Crustaceans, endowed with 

 the faculty of vision, were first placed at the bottom of the 

 primeval seas as at the present moment. 



" Thus we find, among the earliest organic remains, an 

 optical instrument of most curious construction, adapted to 

 produce vision of a peculiar kind, in the then existing repre- 

 sentatives of one great class in the articulated division of the 

 animal kingdom. We do not find this instrument passing 

 onwards, as it were, through a series of experimental changes, 

 from more simple into more complex forms ; it was created, 

 at the very first, in the fulness of perfect adaptation to the 

 uses and condition of the class of creatures to which the kind 

 of eye has ever been, and is still, appropriate." 



Ova. All Crustacea are produced from fertilized ova, 

 which the female, after they have passed from the oviduct, 

 continues to carry about with her until they have attained a 

 certain amount of development. Various are the appendages 

 employed for this purpose; perhaps no example will be more 

 generally known than the one afforded by the common lobster 

 when " in pea." 



Metamorphoses. The young do not, on their liberation 

 from the ova, present a miniature resemblance to the species 

 to which they belong. The contrary opinion was formerly 

 entertained, and it was even regarded as one of the charac- 



