328 PALAEONTOLOGY OF ILLINOIS. 



is the steady growth of a tendency to provide for all the re- 

 cognizable stages in the development or evolution of organic 

 nature.* Perhaps the principal cause for this tendency is found 

 in the fact that now, more than ever before, naturalists con- 

 fine themselves to the study of special groups, and it is well 

 known that specialists, at any rate modern ones, make species 

 and genera upon small provocation. Their great familiarity 

 with the minor details of structure may perhaps magnify the 

 points of difference, but, on the other hand, we must not for- 

 get that this same familiarity gives them an insight into the 

 relative values of structural variations which the ordinary stu- 

 dent cannot possess. As good illustrations I have but to men- 

 tion the admirable system recently proposed by Zittel for the 

 classification of the Sponges, Haeckel's work on the RADIOLARIA, 

 and Wachsmuth and Springer's excellent work on the PAL^EO- 

 CRINOIDEA. Some of the older authors too, who have kept pace 

 with the remarkable progress of natural history, and have 

 adopted modern methods of investigations, offer striking in- 

 stances of the "tendency." Take for instance Hall's recent 

 works on Devonian Fossils, Barrande's last, and De'Koninck's 

 magnificent contributions to the Carboniferous fauna of Bel- 

 gium. Even more striking in this respect is Davidson's supple- 

 mentary work on British Brachiopoda. He was, perhaps, the 



* Every classification must, to a certain extent, be artificial and arbitrary, since it is 

 impossible to draw up a system that will follow out all the intricacies of evolutional ex- 

 istence. Again, the "vanishing lines" of nature forbid our looking for sharply defined 

 groups or species, consequently our divisions are necessarily more or less arbitrary _ 

 In practice, species gradate almost insensibly by means of intermediate species and 

 varieties into each other, while the same instability of characters gradually effaces the 

 separating lines between genera and groups of higher rank. Some authors upon find- 

 ing the intermediate links between closely allied species are not content with the 

 knowledge thereby gained, but make use of it by reducing the "excessive" number of 

 species. I take a different standpoint, and would like to see every recognizable stage 

 in the evolution of organic beings distinguished from the next by its own name. Now, 

 whether this stage is ranked as a variety or species, is a matter of very little moment 

 to me, since in most cases, the palaeontologist at any rate, finds it almost a hopeless 

 task to define just what constitutes a variety and what ought to be called a species. 

 My idea of a genus is a more or less arbitrary group of species, having intimate natural 

 relations to each other. In other words, a greater or less number of species having 

 certain structural peculiarities in common. Sub-genera I hold are a useless encum- 

 brance of nomenclature, as there is very little, if any, need for intermediate divisions 

 between species and genera. So far as I can see they only serve to destroy the uni- 

 formity of classification in giving an undue rank and latitude to the genera to which 

 they belong. In many cases these subdivided genera may be said to assume the rank 

 of families. 



