190 



LECTURE XV 



conclusion that the Crustacea undergo no metamorphosis, and that 

 the contrary evidence adduced by Slabber and Mr. Thompson must 

 depend on some erroneous observation. 



The opposite conclusions of both parties from the phenomena 

 afforded them by the solitary species examined, may be compared 

 with analogous generalisations which might have been drawn, in re- 

 ference to the class of Insects, by the observer of the development of a 

 cock-roach on the one hand, and by the observer of the metamorphoses 

 of a butterfly on the other. As reasonably might the one, after de- 

 tailing the progressive development of the orthopterous insect, broach 

 the inference that insects underwent no other metamorphosis than the 

 gradual acquisition of wings ; and, with equal reason, might the other 

 observer of the wonderful changes of the lepidopterous insect aflfirm 

 them to be characteristic of all insects. It needs only that each 

 theorist should question the reality of the other's observation to make 

 the parallel complete. The failure of both to arrive by so short and 

 easy a route at the entire truth, inculcates the necessity of acquiring 

 a sufficient foundation, by careful and extensive induction of facts, 

 before proceeding to erect the superstructure of general theory. 



With regard to the metamorphosis of the common crab, valuable 

 testimony in confirmation of Mr. Thompson's discovery has been 

 contributed by Capt. Du Cane R. N.* This gentleman obtained crabs 

 with ova under their tails in the month of December, from which the 

 larvae were produced in the months of March and April : the form of 

 the larva at this period is shown 2iXfig. 94. Soon after exclusion this 

 larva casts off its envelope and assumes the 

 appearance represented in fig. 95., which 

 closely corre- 

 sponds with that 

 zoaeiform Crus- 

 tacean, whose 

 further changes 

 were witnessed 

 by Mr. Thomp- 

 son, and which 

 he had assured 

 himself was an 

 early or larval 

 state of a common crab. The last form which immediately precedes 

 the assumption of the mature characters corresponds, according to 

 Dr. Thompson, with that of the genus Megalopa. 



The additional evidence afforded by Capt. Du Cane in proof of the 



Larva of Crab. 



Larva of Crab. 



Annals of Natural History, 1 839, p. 438. 



