1893.] Bacteriology in its General Relations. 1055 
in twenty-four hours. We can therefore accumulate the effect 
_of certain external influences upon an almost infinite series of 
generations within a limited time. The time element, which 
in higher forms of life often necessitates the extension of expe- 
riments over a period of years owing to the relative slowness 
of reproduction, is here. minimized to such an extent as to be 
brought entirely within the limitations of a single observer. 
By a rapid successive transference of cultures to fresh media, 
we can secure the effect of an experiment covering an 
immense number of generations within a limited space of 
time. Of course, the absence of sexuality in the reproductive 
process narrows the sphere of investigation, but there is no 
valid reason why as valuable results may not be obtained by 
experimental work on problems of variation and heredity as 
have been already accomplished with asexuallv propagated 
plants, like the sugar cane, banana, and potato. The objection 
that might be raised, that the morphological and physiologi- 
cal characters are more plastie, and therefore more easily 
modified than higher specialized forms of life, seems to be 
answered when we take into consideration the number of gen- 
erations that intervene between the original type and the 
establishment of a pronounced variety. The gardener is able 
“to modify the constitution of his plant by cultivating it under 
special conditions for a few years to such an extent that he 
produces a horticultural variety in a limited number of gener- 
ations. The bacteriologist in his “ microscopic horticulture d 
finds it far more difficult to modify his species to the same 
extent in a limited number of generations. : 
The ease with which experimental conditions can be modi- 
fied in the manipulation of bacteria is also a valuable factor. 
The physical and chemical environment can be so 
rigidly controlled that the variability of conditions which is 
so disturbing a factor in experimental work on higher forms 
is practically excluded. 
These are some of the evident advantages that bacteria pos- 
sess for experimental research in evolutionary biology. It 
may be proper in this connection to state a few of the results 
-which have been obtained in this field and which bear more 
