312 VICTOR E. SHELFORD. 



increased and the time between them lengthened until the animals 

 took up a characteristic position. The experiment was repeated 

 with five other individuals confined in the center positions (B of 

 Fig. 36) and again with five more confined in the dark portion 

 (C of Fig. 36). The last 20 readings of each series A, B, C, were 

 averaged to give the results shown in Figs. 38 and 39. In Fig. 

 38, we note that (i) the animals living under the stones in rapid 

 water selected the darker portion of the gradient box; (2) Hy- 

 dropsyche which is found under and on stones (in algae) is less 

 negative to the intensities used than those always under the 

 stones; (3) Goniobasis which always lives on stones is more 

 positive than any of the others; (4) Cambarus in these undis- 

 turbed conditions showed a slight excess percentage in the 

 strongest light; (5) the darters were indifferent remaining in the 

 third in which they were confined. 



The animals of the pool community behaved very differently. 

 Notropis was quite positive while the other fishes and Calopteryx 

 which are associated with vegetation were quite negative. The 

 Mollusca and Macromia were inactive. 



VI. SUMMARY AND DISCUSSION. 



Figs. 40 and 41 are introduced to show the character of the 

 agreement and disagreement in the rapids community and the 

 fact that the pool community is different and remains unsolved. 

 Noting first Fig. 40, we see a noteworthy agreement in reaction 

 to bottom (a preference for hard bottom which means avoidance of 

 sand] and to current. Those living on or under stones (including 

 Hydropsyche were found largely on stones in alga?) were under 

 stones in general darkness. Goniobasis which lives on stones was 

 found on stones in the experiments. Etheostoma and Cambarus 

 which live among stones are found among stones. Thus we have 

 vertical disagreement in the matter of relation to bottom. 

 Turning lo reactions to light we find a comparable difference. 

 Animals living beneath stones show a preference for weak light, 

 those on stones medium light, and those among stones strong 

 light. If we were to study out the community in full we would 

 find that reactions to many other factors would be of importance. 

 The formation of associations (Wodsedalek, '12) no doubt is of 



