UNITY OF TYPE IN VERTEBRATES 55 



speaking, only a single animal." 1 The "single animal" is 

 simply the generalised type. 



Having laid down his two principles Geoffrey goes on to 

 apply them to the difficult case of the comparison of the 

 skeleton of fish with the,' skeleton of the higher Vertebrates. 

 " My present task is to demonstrate that there is no part of 

 the bony framework of fishes that cannot find its analogue in 

 the other vertebrated animals." It seems at first sight that 

 many bones are peculiar to fish, formed expressly for 

 performing the functions which fish do not share with higher 

 animals. These are the bones connected with respiration 

 the operculum, the branchiostegal rays, the branchial arches, 

 and others. That the peculiar bones should be connected 

 with the respiratory functions is only natural, for the contrast 

 between fish and higher Vertebrates is essentially a contrast 

 between water-breathing and air-breathing animals. Con- 

 sidering first the general form of the skeleton in fish, we are 

 met at once with a difficulty ; there is no obvious homologue 

 in fishes of the neck, the trunk, and the abdomen of higher 

 animals. What apparently corresponds to the trunk is in 

 fishes crowded close up under the head. But, after all, it is 

 not of the essence of the vertebrate type to have the trunk 

 and the abdomen attached at definite and invariable distances 

 along the vertebral column that is a notion surviving from 

 the anatomy which made man its type. The "trunk" differs 

 in position according to the class, in quadrupeds, birds, and 

 fishes (p. 9). Now, says Geoffrey, allow me this one 

 hypothesis, that the trunk with its organs can, as it were, 

 move bodily along the vertebral column, so as to be found in 

 one class near the front end of the vertebral column, in 

 another about the middle, and in a third near the end, then 

 I can show you in detail that the constituent parts of this 

 trunk are found in all classes to be invariably in the same 

 positions relatively to one another (p. 10). It is important 

 to note this hypothesis of a "metastasis" which Geoffroy 

 makes, for it is the key to the understanding of many of the 

 far-fetched homologies which he tries to establish. It is, of 

 course, clear that this hypothesis is in formal contradiction 



1 Etudes progressives (Pun Naturaliste, p. 50, Paris, 1835. 



2 Philosophic Anatomique., i., Introduction, p. I. 



E 



