.324 THE MECHANICAL FUNCTIONS. 



they may differ in subordinate details of organi- 

 zation, are yet constructed on one uniform prin- 

 ciple, and appear like varied copies from the same 

 original model. In no instance do they present 

 structures which are altogether isolated, or can be 

 regarded as the results of separate and independent 

 formations. 



In proceeding from the contemplation of the 

 structures of articulated to those of vertebrated 

 animals, we appear to pass by a rapid excursive 

 flight, from one great continent to another, sepa- 

 rated by an immense gulf, containing no inter- 

 mediate islands from which we might gather 

 indications of these tracts of land having been 

 originally connected. At the very first sight, in- 

 deed, the general fabrics of these two descriptions 

 of animals appear to have been constructed on 

 opposite principles ; for in the one, as we have 

 already seen, the softer parts are internal, and are 

 enclosed in a solid crust, or shell, or horny covering, 

 answering at once the purposes of protection and 

 mechanical support, and furnishing extensive sur- 

 faces for the attachment of the organs of motion. 

 But in the Vertebrata, the solid framework which 

 serves these purposes occupies, for the most part, 

 an internal situation, constituting a true jointed 

 skeleton, which is surrounded by the softer organs, 

 and to which the muscles, destined to move their 

 several parts, are attached. The office of external 

 defence is entrusted solely to the integuments, and 

 their appendages. Such is the general character 

 of the arrangements which nature has here adopted ; 

 from which, however, she has occasionally deviated 

 with respect to some important organs of extremely 



