HISTORY OF ZOOLOGY 15 



The Nature of Species. — In nature we find only separate animals: 

 how comes it that we classify them into larger and smaller groups? Are 

 the single species, genera, and the other divisions fixed quantities, as it 

 were fundamental conceptions of nature, or a Creator's thoughts, which 

 find expression in the single forms? Or are they alistractions which man 

 has introduced into nature for the purpose of making it comprehensible 

 to his mental capabilities? Are the specific and generic names only 

 expressions which have become necessary, from the nature of our mental 

 capacity, for the expression of relationships in nature, which in and for 

 themselves are not immutable, and hence can undergo a gradual change? 

 Practically speaking, the problem reads: are species constant or change- 

 able? WTiat is true for species must necessarily be true for all 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 con- 

 ception of species was John Ray. In the attempt to define a species he 

 encountered difficulties. In practice, animals which differ little in 

 structure and appearance from one another are ascribed to the same 

 species; this practical procedure cannot be carried out theoretically; for 

 there are males and females of the same species which differ more from 

 one another than do the representatives of different species. Ray reached 

 the genetic definition when he said: for plants there is no more certain 

 criterion of specific unity than their origin from the seeds of specifically 

 or indi\'idually like plants; that is to say, generalized for all organisms: 

 to one and the same species belong individuals which spring from similar 

 ancestors. 



The 'Cataclysm Theory.' — With Ray's definition an uncontrollable 

 element was brought into the conception of species, since no systematist 

 usually could know anything, as to whether the representatives of the 

 species before him sprang from similar parents. It was therefore only 

 natural that the conception of species put on a religious garb, since by 

 resting upon theological ideas it found a firmer support. Linnosus said: 

 "Tot sunt species quot ab initio creavit infinitum Ens." Linna^us's 

 definition showed itself untenable, as soon as paleontology began to make 

 accessible a vast quantity of extinct animals preserved as fossils. Cuvier 

 proved that these fossils were the remains of animals of a previous time. 

 Just as the formation of the earth's crust by successive layers made possible 

 the recognition of different periods in the earth's history, so paleontology 

 showed how to recognize different periods in the vegetable and animal 

 world of life on our globe. Each geological age was characterized by a 

 special world of animals; and these animal worlds differed the more from 



