INTRODUCTION. 3 



animal were gross conceptions of the ignorant and had 

 no morphological meaning. These expressions merely indi- 

 cated the position which an animal assumed in locomotion 

 relative to the earth, and were in this sense convertible 

 terms, since many invertebrate animals prefer to swim on 

 their "backs," while some fishes also do the same, others 

 again (flat-fishes, Pleuronectidae) swimming on their sides. 



The surfaces of the body in the respective groups having 

 been thus reconciled, Saint-Hilaire proceeded to a detailed 

 comparison between an insect and a vertebrate. The chiti- 

 nous rings of an insect represent the vertebras of the higher 

 animals. The viscera of an insect are thus enclosed within 

 its vertebral column, and this condition is compared with 

 what is found in turtles and tortoises where the carapace is 

 fused with the vertebral column. It was necessary to con- 

 clude, and Saint-Hilaire did not hesitate to do so, that the 

 legs of insects were equivalent to the ribs of Vertebrates. 



It was not the intention of Saint-Hilaire to speculate 

 concerning the ancestry of the Vertebrates, for this would 

 have been impossible at the period in which he did his 

 work, but he merely wished to demonstrate the truth of 

 his principle of the unity of the plan of composition of the 

 animal body. He had therefore no reason to be satisfied 

 with having shown, as he believed, how the Insects could 

 be regarded as possessing a structure essentially similar to 

 that of the Vertebrates, but he had next to show how his 

 principle could be applied to other groups, above all to the 

 group of the Cephalopod Molluscs (squids, cuttle-fish, etc.). 

 This happened in the year 1830, and it precipitated the 

 celebrated and somewhat bitter dispute between the great 

 Cuvier and Saint-Hilaire with regard to the question of 

 " types." While Saint-Hilaire only recognised one uni- 

 versal type, Cuvier arranged the different groups of animals 



