1054 ` The American Naturalist. [December, 
cal laws have been but few. Valuable data have however, often 
been obtained in an incidental way. The possibilities, which 
this group of organic life offers for the study of many of these 
general problems, are so noteworthy that it seems worth while 
to call attention to them more in detail. Inselecting forms 
for the study of certain questions, the biologist chooses, as far 
as possible, the primitive generalized types upon which to 
base his observations. Specialization of form and function 
complicate the conditions and render ‘it more difficult to 
apprehend the fundamental truth. In this respect bacteria 
occupy a unique position. Morphologically considered, they 
are a lowly organized and generalized type, while functionally, 
they possess a marked degree of specialization. 
With our present appliances but little difference can be 
detected in form between many species that possess widely 
divergent physiological functions, so that species are often 
found that are morphologically similar and their dominant 
physiological function may be expressed either in pigment 
production, fermentative action, or in an infectious malady. 
A certain degree of adaptability in an organism is also nec- 
essary if we are to subject it to prolonged experimentation. 
Many plants and animals are so susceptible to any modifica- 
tion in their surroundings that they cannot well be utilized 
for purposes of experimentation, a slight change often being 
sufficient to produce a cessation of the vital functions. Bacte- 
ria possess an adaptability not to be found in any other 
class of organisms. With a large proportion of these forms, 
the range in temperature of the limits of growth far exceeds 
any of the higher forms of life. 
The majority of species are able to vegetate between 10° C. 
and 50° C. while exceptional ones grow at the freezing point 
and others thrive at a temperature of 70° C. This tenacity 
of life, far surpassing all other forms of animate nature, is as 
distinctive in the chemical as in the physical environment of 
these germs. 
Another peculiar characteristic, that mindu them of espe- 
-cial value from an experimental standpoint, is their rapidity 
-of multiplication. A single cell is the progenitor of millions 
