36 SIXTEENTH REPORT. 



gyrina in group 2, and another of Strophitus edentulus in group 3, showed 

 how stray specimens might often be found at a considerable distance from 

 their associates of the same species, and the pools might have been largely- 

 populated by similar w^andering migrants. 



The temporary swamps present a still more difficult problem. The shells 

 in them were found nowhere else around the lake, and they were not connected 

 with each other or with the lake even by temporary streams. It must have 

 been that their comparative tolerance of dessication enabled them, or their 

 eggs, to be carried easily, by birds, etc., from pool to pool. 



V. The Vegetation. The swamps and pools usually contained a greater 

 amount of vegetation than did any portion of the lakes with the exception 

 of the weed zones, and it was also of a different character. This, of course, 

 had a marked effect or the deposition of humus and on the amount of humic 

 acids in the water. However, the effects of this were not especially brought 

 out. 



VI. The Temperature. During the summer, the temperature of the pool 

 was considerably higher than those of the littoral zones of the lake, and 

 must have been lower in winter. This was especially true of the beach 

 pools, exposed to the direct sun on the treeless, sandy beaches. This must 

 have had a considerable effect on the shells themselves, and on their food 

 supply. 



VII. Proportion hetween Area of Surface and Total Volume. The larger 

 area of the surface of the pools, in proportion to the total amount of water, 

 was in very marked contrast to that of the lake; this strongly effected the 

 preceding factor, and also allowed greater aeration and also greater evapora- 

 tion in the former habitats. The greater evaporation apparently resulted 

 in high mineral content; the iron deposits of beach pools were apt to be 

 quite prominent and must have been attended by higher concentration of 

 other materials, altho this was not especially tested. 



b. Conclusion. 



In conclusion, it might be remarked that the beach pools of Douglas 

 Lake gave a marked example of animal succession. As the waves and cur- 

 rents built up the spits and bars, the environment changed, and along this 

 differentiation came changes in the moliuscan fauna. The fauna, however, 

 had little effect on these initial metamorphoses, but after the separation, 

 and during it, there must have been considerable competition between the 

 old and the new forms. In addition, they must have become considerable 

 factors in the small pools, by changing the gaseous and organic content, 

 etc. In this succession, the larger species of the lake habitats appeared to 

 be the first to go, while the secondary forms appeared to be the pioneers. 

 Part of this latter, apparent succession maj- have been due, however, to 

 the large size of the climax pool as compared with the lagoons. 



Despite the considerable effects of the animals on their environment, 

 the physical factors were the main causes of the change from lake to pool, 

 while the vegetation appeared to be the main factor in the change from 

 pool to temporar}' swamp. Animal ecology, even as compared to plant 

 ecology, seemed to be much more the study of habitats than of associa- 

 tions. 



Colorado College, Nov., 1914. 



