Classification of Animals. 15 



reconstruction of the fossil fragments, so the recognition of 

 extinct forms aided in the arrangement of their living successors, 

 filling up some of the morphological gaps. 



(4) The work of Cuvier must always be associated with the 

 idea of the "correlation of parts" that the organism is a 

 morphological unity. Certain characters are invariably corre- 

 lated, others as invariably exclude one another; in short, the 

 part is of a piece with the whole. 



The anatomical and palaeontological foundations of 

 classification were recognized by Cuvier, but there is a 

 third foundation, namely in embryology. It 

 seems fair to credit Von Baer (1792-1876) 

 with laying this third foundation, not so 

 much because he confirmed on embryological 

 grounds the four embranchements of Cuvier which was 

 a mistake in detail but because he saw clearly that the 

 study of development was a sure clue to relationship. 

 We find the same idea in the work of Johannes Miiller 

 (1801-1858), whose genius influenced almost every 

 department of zoology; in Vaughan Thompson's dis- 

 covery of the Crustacean nature of Barnacles ; and con- 

 spicuously in Kowalewsky's account of the development 

 of Ascidians and the lancelet (1866). 



The pedigree of a noble stock, and the relationships 

 between the different branches of the family, may be 

 conveniently represented by a number of Genealogical 

 diverging and forking lines, and these may Tr> ees. 

 readily assume a more or less artistic tree-like arrange- 

 ment, which has certainly the merit of vividness. 



It is certain that, before the Theory of Descent was 

 accepted or even discussed, genealogical trees were 

 used to represent possible relationships among human 

 races, or possible affinities among animals. It was used 

 as a "graphic" way of expressing classification, and 

 was true just in proportion as the classification was 

 true. The naturalist-traveller Peter Pallas was one of 

 the first to use it to express affinities among animals, 

 though it is possible he saw a deeper meaning in his 

 symbol. 



But when the Theory of Descent took hold on men's 

 minds, the genealogical tree became more than a graphic 



