INTRODUCTION. 



The first zoologist to put forward, in a definite manner, 

 the view of the existence of a direct relationship between 

 Vertebrates and Invertebrates was the celebrated Etiexxe 

 Geoffroy Saixt-Hilaire. 



It would appear that without anv previous zoological 

 training, having been brought up as a botanist and 

 mineralogist, he was appointed Professor of \'ertebrate 

 Zoology at the Museum of the Jardin des Plantes in the 

 year 1793, being then twenty-one years old. His col- 

 league as Professor of Invertebrate Zoology was the no 

 less distinguished Lamarck. 



Saint-Hilaire's study of the comparative anatomy and 

 osteologv of the different groups of \'ertebrates — Fishes, 

 Amphibians, Reptiles, Birds, and ^Mammals — impressed 

 him strongly with the conviction that, in spite of the 

 many obvious contrasts existing between these animals, 

 they are nevertheless essentially constructed upon the 

 same plan, the same parts recurring in all the groups 

 under a more or less altered form. ^Moreover, such 

 observations as, for example, that the bones of a fish's 

 skull can be more readily compared with the bones of an 

 embryonic mammalian skull than with those of the adult, 

 and that the bones of a bird's skull are separated in the 

 young by sutures just as they are in the skull of a 

 mammal, led him to frame his three great principles in 



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