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SCIENCE. 



[N. S. Vol. VIII. No. 196. 



ical doctrines which we believe thoroughly 

 but which we do not allow to interfere with 

 our daily life, or, as far as botany is con 

 cerned, has our belief modified the manner 

 in which we treat what we call species ? The 

 mere fact that we now recognize that species 

 have been derived from other species, and are 

 on the way to develop into still other species, 

 would naturally lead us to be more liberal 

 in our treatment of them systematically 

 than in the days when variation was almost 

 a crime against the Almighty. Certainly, 

 with evolution as a key to guide us, our 

 conceptions of genera and orders ought to 

 be far more scientific than they were. 



A species has been defined as a perennial 

 succession of like individuals and, although 

 no definition is perfect, I doubt whether a 

 better definition of species has ever been 

 invented. It is a peculiarity of definitions, 

 however, that they all need to be defined. 

 In the present case we must be told what is 

 meant by the word perennial and what is 

 meant by like. To the pre-Darwinian, per- 

 ennial, of course, meant for all time. By 

 the early Darwinians we are not told 

 whether by perennial they meant a hundred, 

 a thousand or a million years, but until, 

 at least, we know approximately what is 

 meant we must still ask how long must be 

 the succession of like individuals to estab- 

 lish a good species. Otherwise the whole 

 matter of the distinction between a race 

 and a species cannot be settled practically. 

 If there is nothing definite in writings of 

 the time of Darwin to explain the limits of 

 the perennial succession, we should bear in 

 mind that the object then was to bring out 

 boldly the salient points of evolution as 

 governed by natural selection, and the illus- 

 trations used were taken almost exclusively 

 from the higher animals and plants in 

 which the lives of individuals are of such 

 duration that it was impossible to obtain 

 accurately the records of a large number of 

 generations in any case. Enough was shown 



and cited to show from the records of com- 

 paratively few generations a general ten- 

 dency which it was assumed would be 

 confirmed could the geological record be 

 followed, and we can suppose that, so far 

 as they considered the question at all, 

 the early Darwinians took it for granted 

 that the perennial succession needed to 

 establish a species covered very long inter- 

 vals of time. While one need not object to 

 this method of reasoning, it is plain that 

 the practical question of when a race or 

 variety ceases to be a race and becomes a 

 species was left open, and it is questions of 

 this sort which the systematist is constantly 

 called upon to answer. 



What could be learned only slowly and 

 fragmentarily from observations and ex- 

 periments on higher plants and animals 

 might, perhaps, be learned much more easily 

 could one experiment with organisms whose 

 cycle of life is completed with great rapidity. 

 For this purpose one might suppose that 

 nothing could be better than bacteria, which 

 are easily managed in the laboratory and 

 whose development takes place with such 

 rapidity that it is possible for the experi- 

 menter to watch the course of hundreds 

 or even of a thousand generations in a com- 

 paratively short time. 



The advantage to be expected from study- 

 ing forms in which the development is very 

 rapid is, however, made difiicult for purposes 

 of comparison by their extreme simplicity 

 and the difficulty and, at times, impossibil- 

 ity of finding sufficiently marked morpho- 

 logical characters to guide us and, in the 

 absence of such characters, the bacteriolo- 

 gist is often forced to base what he calls his 

 species on physiological characters, includ- 

 ing in that term zymotic and pathological 

 action. By botanists; who are not specially 

 bacteriologists, the so-called species of bac- 

 teria are not admitted to be species in the 

 proper sense. Whether scientifically con- 

 sidered they are not as legitimately species 



