HISTORY AND NATURAL HISTORY 21 



could crawl about readily. When a current was 

 stirred in the water the isopods from the streams 

 usually headed against it; but those from ponds were 

 more likely to head down current, or to be indif- 

 ferent in their reaction to the current. The behavior 

 of the two types was sufficiently different so that at 

 first I thought that stream and pond isopods repre- 

 sented different species, but the specialist at the 

 National Museum assured me that all belonged to 

 the species appropriately called Asellus communis, 

 the commonest isopod of our inland waters. 



Rather cockily I reported after a time to my in- 

 structor that I had gained control of the reaction of 

 these animals to a water current. By the judicious 

 use of oxygen in the water, I could send the indif- 

 ferent pond isopods hauling themselves upstream, or 

 I could reduce the stream isopods to going with the 

 current. I had not reckoned with another factor that 

 presently caught up with me. 



After a winter in the laboratory it seemed wise as 

 well as pleasant to take my pan out to a comfortable 

 streamside one sunny April day, and there check the 

 behavior of freshly collected isopods in water dipped 

 from the brook in which they had been living. To 

 my surprise, the stream isopods, whose fellows all 

 winter had gone against the current, now went 

 steadily downstream or cut across it at any angle to 



