12 THE DEPTHS OF THR SEA. [CHAP. i. 



has yet been observed of one species passing through 

 a series of inappreciable modifications into another. 

 Every species appears to have an area of maximum 

 development, and this has been called the metropolis 

 of the species ; and practically we must employ the 

 same methods in investigating the laws of its distri 

 bution as if we still regarded it as having been 

 specially created in its metropolis. 



It is the same in dealing with the law of represen 

 tation. Accepting an evolution doctrine, we should 

 certainly regard closely allied or ' representative ' 

 species as having descended comparatively recently 

 from a common ancestry, and as having diverged 

 from one another under somewhat different conditions 

 of life. It is possible that as our knowledge increases 

 we may be able to trace the pedigree of our modern 

 species, and some attempts have already been made 

 to sketch out the main branches of the universal 

 genealogical tree; 1 but practically we must continue 

 to accord a specific rank to forms which exhibit 

 characters to which we have been in the habit of 

 assigning specific value. 



"Every species has three maxima of develop 

 ment, in depth, in geographic space, in time. In 

 depth, we find a species at first represented by few 

 individuals, which become more and more numerous 

 until they reach a certain point, after which they 

 again gradually diminish, and at length altogether 

 disappear. So also in the geographic and geologic 

 distribution of animals. Sometimes the genus to 

 which the species belongs ceases with its disappear 

 ance, but not unfrequently a succession of similar 



1 Ernst Haeckel, op. cit. 



