ECOLOGICAL PAPERS. 131 



plexes end higher in the autumn than they start in the spring — a 

 condition which must be due to the effect of winter kiUing. 



SUMMARY. 



The results so far obtained in this investigation may be briefly 

 summarized as follows : 



(1) The phenomena of seasonal succession holds, both in 

 regard to succession of species and to the number of individuals 

 in a species. 



(2) The position of dominance in point of numbers and of 

 distribution is held by crustaceans in the spring and autumn, with 

 Ascllus communis as the dominant species; and by molluscs in 

 midsummer, with Lymnaca reflcxa dominant. 



(3) The most crowded habitat is on the bottom along the pond 

 margin in spring, on the surface or in the mid-depths of the deep- 

 est water in midsummer, and near the green water plants in 

 autumn. 



(4) The forms tend to distribute themselves over the whole 

 pond, but are much more restricted during part of the year, espe- 

 cially during the breeding season. 



The external factors influencing seasonal succession may be 

 summarized as : temperature, amount of water, chemical compo- 

 sition of water, amount and character of food and the physical 

 condition of the habitat. In seasonal succession the dynamic 

 effect of the animals themselves upon their own habitat is not 

 nearly so marked as in ecological succession; yet this dynamic 

 effect can be demonstrated to be present so that the phenomena 

 of seasonal succession may be regarded as the cyclic or slightly 

 spiral process, by means of which ecological succession is carried 

 on. It therefore presents a minute unit for the study of the gen- 

 eral succession problem. While no attempt has been made as yet 

 to analyze the causes of seasonal succession into their ultimate 

 factors, still, by careful quantitative collecting of animals and by 

 complete chemical analysis of the pond water we may hope to find 

 these factors. At least we can approximate them with much more 

 certainty than in ecological succession, where the time element 

 presents a great complication. 



The writer is indebted to Dr. \*. E. Shelford for help and kindly 

 criticism during the continuance of this work. 



