HISTORY OF ZOOLOGY. 2$ 



division into seven branches, just as they have histor- 

 ically developed, has been adopted. 



HISTORY OF THE THEORY OF DESCENT. 



Importance of the Subject Before we close the his- 

 torical introduction we must consider the historical develop- 

 ment of a question whose importance might on a super- 

 ficial examination be underrated, but which from a small 

 beginning has grown into a problem completely dominating 

 zoological research, and has busied with its consequences 

 not only zoologists, but all circles interested in science 

 generally. I refer to the question of the logical value of 

 the systematic terms species, genus, family, etc. 



The Nature of Species. In nature we find only 

 separate animals : how comes it now that we classify them 

 into larger and smaller groups? Are the single species, 

 genera, and the other divisions which the systematist distin- 

 guishes, fixed quantities, as it were fundamental conceptions 

 of nature, or a Creator's thoughts, which find expression in 

 the single forms? Or are they abstractions which man has 

 brought into nature for the purpose of making it compre- 

 hensible to his mental capabilities? Are the specific and 

 generic names only expressions which have become neces- 

 sary from the nature of our mental capacity, for the grada- 

 tion of the circles of relationship in nature, which in and 

 for themselves are not immutable, and hence can undergo 

 a gradual change ? Practically speaking, the problem 

 reads: are the species constant or changeable? What is 

 true for the species must necessarily be true for all the 

 other categories of the system, all of which in the ultimate 

 analysis rest upon the conception of species. 



Ray's Conception of Species. One of the first to 

 consider the conception of species was Linnaeus's prede- 

 cessor, the Englishman John Ray. In the attempt to define 

 what should be understood as a species he encountered diffi- 

 culties. In practice, animals which differ little in structure 

 and appearance from one another are ascribed to the same 



