398 Evolution and Adaptation 



ing water. If other moving paramoecia 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 paramoecia 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 



