360 
tion of new characters, the environment 
presses very closely upon these unicellular 
forms and they respond more directly to it. 
Generation follows generation with start- 
ling rapidity, elimination of the unfit pro- 
ceeds rapidly, the struggle for existence is 
more severe because of the enormous num- 
bers concerned. In the bacteria we ought 
almost to be able to see the actual process 
of evolution since we can place under ob- 
servation untold numbers of generations as 
compared with the comparatively few gen- 
erations of multicellular forms which we 
are able to observe. Eons of multicellular 
time are literally compressed into a few 
unicellular days. 
Because of this lack of the regulating 
infiuence of reproduction and the greater 
influence of the environment and the rapid- 
ity of reproduction, we might expect to 
find among unicellular organisms a series 
of intergrading forms without divisions 
into groups that resemble the species of 
higher forms. But thisisnotso. Among the 
protozoa differentiation has followed definite 
lines, and classes, orders, families, genera 
and species are well marked. The species 
of protozoa are, for the most part, based 
on morphological differences, it is true, but 
among the bacteria a morphological basis 
of classification fails us. The only morpho- 
logical differences are the three main di- 
visions as to shape, the round, the rod and 
the spiral, with slight modifications as to 
size, arrangement of flagella, formation of 
spores, chemical composition, ete. But in 
spite of this lack of morphological basis for 
classification we find as distinct groups as 
among the protozoa. As the Winslows 
say :° 
Typhoid germs descend from typhoid germs, 
tubercle bacilli from tubercle bacilli. The same 
3 Winslow and Winslow, ‘‘Systematic Relation- 
ships of the Coccacer,’’ p. 1, John Wiley and 
Sons, 1908. 
SCIENCE 
[N.S. Vou. XXXV. No. 897 
yellow coccus falls on gelatin plates exposed to 
the air all over world. The same spore-forming 
aerobes occur in every soil, the same colon bacilli 
crowd the intestines of animals and man in every 
clime. These fundamental types can not be trans- 
formed into each other. 
And yet to a considerable extent these 
fundamental types are based not on mor- 
phology, but upon physiological differ- 
ences. 
It is among the bacteria that for the first 
time among living forms we find physiolog- 
ical differences made a basis for classifica- 
tion. Are physiological properties valid 
criteria for the separation of species? We 
are accustomed to think of morphological 
characters as fairly stable, rather difficult 
to modify, but of physiological characters 
as easily modified and directly dependent 
upon the environment. But have we any 
reason to assume that physiological charac- 
ters are not deep seated also, are not 
stable, fully as much as morphological 
characters? Are we not in reality dealing 
with characters of the same sort as mor- 
phological characters except that we can 
not see their ultimate basis in structure? 
I am not sure but what physiological char- 
acters are even of greater importance than . 
those of form and external appearance, 
especially in such simple and undifferenti- 
ated forms as the bacteria, since they tes- 
tify to deep modifications in the chemistry 
and vital properties of the protoplasm 
itself. It is upon such chemical properties 
as these that these simple organisms de- 
pend for their very existence, and not 
upon a modification of external appear- 
ance. 
But before we can proceed far in the 
use of physiological differences for species 
determination we must be able accurately 
to determine these delicate characters. 
Hitherto this has been impossible because 
of the variable and uncertain culture 
