60 MORPHOLOGY OF THE BACTERIA. 
1. Generic and specific characters.— These have 
been drawn from the dimensions, form, movement 
and evolution of the bacteria. 
The size, which, according to Cohn, is the dom- 
inating distinctive character, is often indetermi- 
nable, even in employing the highest powers. 
Besides, for a great number of neighboring forms, 
the differences of measurement given as distinctive 
are so slight that they cannot serve in practice. 
Thus, according to Dujardin, the Bacterium termo 
has a length of 1, 7 », and the B. punctum of 1, 7 to 
0.6 u. Another difficulty presents itself when we 
examine bacteria formed of several articles. Shall 
we consider the length of a single article or of the 
chain, which consists of a number of articles, a 
number ordinarily variable ? 
The form of the bacteria and their union in 
colonies, also offer differences, which have been 
utilized; but do they depend upon differences truly 
specific, or do they come from foreign influences, 
from phases of development of the same organism? 
Even when one uses these as distinctive specific 
characters, the form is sometimes of little assist- 
ance; since if one refers to the descriptions of 
Dujardin, the Bacterium termo will be found to have 
a cylindrical body swollen in the middle, and the 
B. punctum an elongated ovoid body. 
As to movement, we have seen that the phenom- 
ena of mobility or of immobility sometimes pre- 
sent themselves in the same species, according to 
age or changes in the medium.. 
We have left, the mode of development, the 
