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THE AMEBIC AN NATURALIST [Vol. XLII 



a man were going to revolutionize the world of thought 

 lie certainly, I assumed, would give a definition of the 

 subject he is going to treat. But I could not find a 

 word as to what Darwin meant by species. He writes 

 as if all the world knew what species are. Yet I did 

 find this in the latter part of the last chapter; he says, 

 "And now we shall be freed from the vain search for the 

 undiscovered and undiscoverable essence of the term 

 species." Consequently, here we are, tracing a will-o'- 

 the-wisp. And yet, it seems to me, there must be some- 

 thing that we are all thinking about when we say species. 

 There must be some workable idea. Of course I know 

 that since Darwin's time and the vast accumulation of 

 knowledge in reference to heredity and variation, to say 

 nothing about other physiological aspects proper, our 

 ideas have decidedly changed. Still, as I look over 

 the actual work being accomplished in determining the 

 number and the extent of the species of plants in the 

 world, I do not see that our practise especially differs 

 from the pre-Darwinian practise. We have a theoret- 

 ical idea of what species are, but practically we describe 

 them for the most part in the same pre-Darwinian terms, 

 the really Linnean terms, so far as many of us can, 

 although we do not adhere to the Linnean brevity. 

 Xevertheless, there are some instances in which it seems 

 necessary to deviate from this practise, if we wish to 

 handle our subject in such a way as to make it useful; 

 for I assume, in the first place, that whatever the species 

 concept may be, it is something which enables us to 

 present ideas or problems in a better form than we could 

 otherwise accomplish. At any rate, the idea of a species 

 is a mental tool of some sort. 



To take up the purely phvsiological aspect; possibly 

 it was the study of bacteria which forced this phase of 

 the question upon us for the first time. I judge that it 

 came largely from the fact that bacteria are so very 

 minute. AVhat could one do if he were going to describe 

 the thousands of species of bacteria simply as plants, 



