398 Evolution and Adaptation 
ing water. If other moving paramcecia swim, by chance, 
into this region, they are caught, and as a result an accu- 
mulation of individuals will take place. The more that 
collect the larger will the area become, and thus large num- 
bers may be ultimately entrapped in a region where there is 
formed a substance that, from analogy with other animals, 
we should expect to be injurious. 
The question as to how far these responses of the unicellu- 
lar forms are of advantage to them is difficult to decide, for 
while, as in the above case, the response appears to be 
injurious rather than useful, yet under other conditions the 
same response may be eminently advantageous. In other 
cases, as when the parameecia back away, and then swim for- 
ward again, only to repeat the process, the act appears to be 
such a stupid way of avoiding an obstacle that the reaction 
hardly appears to us in the light of a very perfect adaptation. 
If we saw a higher animal trying to get around a wall by 
butting its head into it until the end was finally reached, we 
should probably not look upon that animal as well adapted 
for avoiding obstacles. 
Bacteria, which are generally looked upon as unicellular 
plants, appear, despite the earlier statements to the contrary, 
to react in much the same way as do the protozoans, according 
to the recent work of Rothert, and of Jennings and Crosby. 
The bacteria do not seem to turn toward or away from chem- 
ical substances, but they collect in regions containing cer- 
tain substances in much the same way as do the protozoans. 
The collecting of bacteria in regions where oxygen is pres- 
ent has been known for some time, but it appears from 
more recent results that they are not attracted toward the 
oxygen, but by accidentally swimming into a region containing 
more oxygen they are held there in the same way as is para- 
moecium in a drop of acid. On the other hand bacteria do 
not enter a drop of salt solution, or of acids, or of alkalies. 
They react negatively to all such substances. Some kinds of 
