494 



THE AMERICAN NATURALIST 



[Vol. LIII 



progression and then they tried anotlier. As Holmes 

 (191(), pp. 157, 158) has said: 



Wiiere there is " error," the organism trios aoaiii, and keeps on doing 

 so until it attains ultimate success. 



This statement does not mean that all achieve success, 

 nor does it necessarily mean that the org^anism possesses 

 any conscious appreciation of means to an end. Cer- 

 tainly, I do not consider that water-striders have such an 

 appreciation. Frequently, on coming' in contact with 

 such obstacles, as have been mentioned, the gerrids came 

 to rest with the side or sides of the body closely applied 

 to the object. This was due to their thigmotactic procliv- 

 ities. They remained in such positions for varying 

 lengths of time and then moved forward again, but 

 usually the direction of }~>rogression was changed. Occa- 

 ssional ly, they remained motionless in such situations 

 until the time set for me to discontinue. my field observa- 

 tions for that particular day. Sometimes individuals 

 crawled under lumps of dried mud, under pieces of drift- 

 wood, or among dead leaves. On a few occasions, a few 

 gerrids jumped into large cracks in the baked mud of the 

 stream bed. AVater-striders getting into sucli places, 

 occasionally remained there, but T never have been able 

 to find them llic day rollowiiig tlie o])S(M-vation. 



It aircjidx \\;\< Ix'cti st.-itcd that xuiie of the gerrids 

 reaclM'(i ihf lai-o(.i- jH.ol oi' ualcr xniie <listance down- 

 stream, and attention also ha> been dii'ected to the fact 

 that on several other occasions, when water-striders had 

 been trapped in stream pools, some of tlieir number were 

 successful in reaching other bodies of water- in the innne- 

 diate vicinity. I have not obx'rvtMl that o(M-i'ids ever 

 weiv Hic(M-rnl in lindin- an-.thrr l.o,K of wa't.-r that wa^ 

 situated raithcr awav than I'oiii-tccii vai'd^. In nonr of 



