1 8 VICTOR E. SHKLFORD. 



2. The Cause of Succession Environment. (a) Area of the 

 Ponds. A comparison of Table I., page 3, with Table XXI., 

 page 17, and a comparison of Table I. of the preceding paper with 

 the map (p. 131 of the preceding paper) show that most of tin- 

 fishes are in ponds of all the available areas of the region, with 

 the exception of several species which are confined to pond I., 

 and which, on account of their numbers, could find no advan- 

 tage in such close quarters. Evidently no part of the aiiM\er 

 lies in the matter of size. 



(6) Depth of the Ponds. A comparison of the records of depths 

 given in Table II., page 4, with Table XXL, page 17, shows 

 a situation parallel to the one with reference to area. Species 

 are in ponds of various depths and are absent from ponds of 

 depths the same as and greater than the ones in which they are 

 found. These ponds are shallower than the waters which many 

 of the species commonly occupy. The matter of depth does not 

 seem to be of importance in the answer to the question. 



(c) Minerals in Solution. The minerals in sqlution in t In- 

 different ponds on October 26, 1909, are given in Table 111. 



(1) Qualitative Differences. The minerals represented in tin- 

 analysis are those normal to w r aters inhabited by fish and probably 

 important to fish. No zinc, lead, aluminum, silver, or coppi-r, 

 metals highly poisonous to fish (Marsh, '10), were found and there 

 is no reason to expect their presence at another time of the year. 1 

 From the qualitative standpoint there is no reason to assign 

 importance to minerals in solution. 



(2) Quantitative Differences. The total solids given in Table 

 III., p. 5, lie between the two extremes given by Marsh, '10, 

 as probably not affecting fish and as "normal" for waters which 

 are known to support fish in numbers. He gives 484 parts per 

 million for the Potomac River and 242 for other fish waters. 

 Nor is a very great seasonal variation to be expected, because 

 most of the animals live through the winter and the vegetation 

 disintegrates very slowly, especially through the cold weather, 



1 Because of the small amount of inwash, this sc-t of ponds afford an IIIIHMM] 

 opportunity for the study of the effect of a varying amount of vegetation n ilu- 

 chemical composition of the water. For a statement of the salts ti-.l up l>\ plant- 

 see Pfeffcr-Ewert, 'oo, page 410. 



