134 PHOTO-MICROGRAPHS. 
reagents, it will be seen that considerable differ- 
ences as to size occur in different individuals of the same 
species and in the same culture medium. 
These facts, which have all been verified by 
the personal observations and experiments of the 
writer, make it evident that the laws of natural 
selection have full play among these lowly mi- 
croscopic plants, as well as among those higher 
in the scale. 
Micrococci are widely distributed in nature ; 
they are constantly floating in the atmosphere in 
a desiccated condition, or in the shape of ultra- 
microscopic germs; and when they are deposited 
in a liquid or upon a moist surface, where the con- 
ditions are favorable for their growth, they quickly 
resume their reproductive activity. 
If a drop of fluid containing micrococci — as, for 
example, a drop of blood from a rabbit, just dead, 
containing the species shown in the figure — be 
added to a transparent “ culture-fluid” such as 
bouillon made from the flesh of a rabbit, in a “ ster- 
ilized”” flask, and this be maintained at a tempera- 
ture suitable for the multiplication of the little 
plant (98° — 100° for the species under consider- 
ation), the fluid will become opalescent or even 
milky in appearance within twenty-four hours, 
as the result of the rapid increase which has taken 
place. The presence of these minute organisms, 
when they are in sufficient abundance, may then 
be recognized by the naked eye. The colored spe- 
cies are especially likely to develop on cooked 
alimentary substances. Cooked potato is a good 
