THE AKCHIPELAGO OP BREHAT. 103 



severance wliicli he had shown, to the study of the 

 lower animals on the borders of the sea. 



In fine, types are the more fixed in proportion as 

 they are more perfect. In those animals which 

 approximate the most closely to this standard, the 

 organism is extremely complicated, and it would 

 seem that nature does not necessarily interfere with 

 essential characters in producing a large number of 

 derivatives. In the Yertebrata, for instance, whose 

 primordial type gives origin to four classes, viz., 

 mammals, birds, reptiles, and fishes, the general 

 plan merely undergoes different secondary modifica- 

 tions. The external forms are changed to facilitate 

 some special mode of locomotion ; the lungs become 

 metamorphosed into branchias, in order to allow of 

 respiration in the water, but yet we find, in passing 

 from the ape, which by its organisation approximates 

 most closely to man, and descending to the lowest 

 type of fishes, that nearly the same functions are 

 fulfilled by a nearly equal number of organs disposed 

 in an analogous if not an identical manner.* Those 

 whose attention has been mainly directed towards 

 the higher animals can form no idea of the extent to 

 which organic degradation is carried; and hence, 

 whenever their inquiries happen to lead them to 



* The AwpJiioxus forms one of the most remarkable exceptions to 

 this general rule. This fish, if we may call it by that name, exhibits, in 

 the Vertebrata, an example of extreme degradation, which we should 

 not have expected to meet with in any but the Invertebrata. It 

 ought therefore to be formed into a special class, if it did not appear 

 strange to constitute so important a subdivision for one or at most 

 only two species. 



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