78 Trans. Acad. Sci. of St. Louis. 



of the former systematists. The Cuvierian Period held sway 

 until the middle of the century when it in turn gave place to 

 a second philosophical reaction with the beginning of the 

 Darwinian Era. 



From the time of antiquity those who concerned themselves 

 with philosophical conceptions of nature regarded animals as 

 constituting a linear series of increasing complexity, or a scala 

 naturae as it was called, and to this conception Linnaeus' 

 system of classification lent great weight. Linnaeus believed 

 that the whole animal kingdom could be arranged in such a 

 series, beginning with the simplest forms and ending with the 

 most complex, the species falling within the genera, the 

 genera within the orders and the orders within the classes, 

 succeeding one another in regular linear gradation. 



In the history of systematic zoology the only name between 

 Linnaeus and Cuvier which need be mentioned here is that of 

 Lamarck who, however, is chiefly distinguished as the founder 

 of a theory of evolution. Lamarck's classification was merely 

 an enlargement and logical development of Linnaeus', but 

 owing to the progress which had been made during the fifty 

 years intervening in the knowledge of animal forms, espe- 

 cially of the lower forms, it contained twice as many classes 

 and ten times as many genera as were recorded by Linnaeus. 

 To him is due the introduction of the terms invertebrate and 

 vertebrate to indicate animals without and those with an axial 

 supporting structure or back-bone. 



In the revolt against the systematists during the latter part 

 of the eighteenth century the study of comparative anatomy, 

 long laid aside for species-description, had its rebirth, and 

 there had arisen as a result of the comparisons made between 

 the different parts of the same organism and similar parts of 

 different organisms two great principles; namely, the Corre- 

 lation of Parts and the Homology of Parts. According to 

 the former it was recognized that an organ is not an isolated 

 thing but that there exists in the body a mutual dependence 

 of all its parts, certain features in one structure being always 

 associated or correlated with certain features in another, as 

 for example, in the teeth and in the digestive tract. The 

 principle of correlation, first formulated by Cuvier, was soon 

 to be carried by him to an extreme in maintaining that from 



