680 THE AMERICAN NATURALIST. [VoL. XXXII. 
how long must be the succession of like individuals to establish 
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 
illustrations 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 comparatively 
few generations a general tendency, 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 suc- 
cession needed to establish a species covered very long intervals 
of time. While one need not object to this method of reason- 
ing, 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 experiments on higher plants and animals 
might perhaps be learned much more easily could one experi- 
ment 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 rapid- 
ity that it is possible for the experimenter to watch the course 
of hundreds or even of a thousand generations in a compara- 
tively short time. ; 
The advantage to be expected from studying forms in which 
the development is very rapid is, however, made difficult for 
purposes of comparison by their extreme simplicity and the dif- 
ficulty, and at times impossibility, of finding sufficiently marked 
morphological characters to guide us; and in the absence of 
such characters the bacteriologist is often forced to base what 
