24 THE TROPISMS 
band or ring whose diameter varies with the size and strength 
of the drop. As the solution diffuses and becomes diluted 
at the center, the ring widens and extends toward the 
center and after a time closes in to form a solid group. If 
two species of protozoa having a different attunement to 
carbon dioxide are used they may form two rings, one out- 
side of the other. 
The tendency of Paramecium to form groups may be 
explained as due, in part at least, to their positive chemo- 
taxis to carbon dioxide. If water containing Parameecia 
is allowed to stand undisturbed for some time most of the 
individuals will be found in 
groups or clusters as if they 
were drawn together by their 
social proclivities or by some 
: object of common interest. But 
Fia. 2.—Showing the positive the cause of the association is 
chemotaxis of Paramcecium to : 
a drop of a weak solution of car- much simpler. As Paramcicea 
es give off carbon dioxide there 
is a greater quantity of it in a region where several 
Paramcecia happen to be associated. Positive chemotaxis 
to this substance tends to keep the cluster together as well 
as to retain other individuals which happen to come into 
the region, until finally most of the Parameecia in the vicinity 
are assembled in the group. Other infusorians, such as 
Oxytricha and Loxocephalus, form associations which are 
not due to carbon dioxide or to any other acids since they 
show no positive chemotaxis to these substances. There 
is probably some substance which produces the grouping, 
although it has not yet been identified. 
The chemotaxis of the lower organisms is to a certain 
extent related to the welfare of the individuals, although in 
only a general sort of way. The negative reaction to strong 

