The Vertical Distribution of the Crustacea. 375 



While, therefore, I find swarms occasionally present, I find 

 also that the Crustacea of lake Mendota are in general distrib- 

 uted with marked uniformity. Marsh ('97, p. 220) finds an ordi- 

 nary variation of ten-fold in the numbers of Diaptomus and an 

 even greater variation in the case of other limnetic Crustacea. 

 With the exceptions already noted the range of variation in 

 lake Mendota has not often exceeded four fold. The number of ob- 

 servations, therefore, necessary to give a fair average for the 

 population of the lake is not so great as that spoken of by Marsh. 

 The examination of my records shows that the general development 

 of the Crustacea can perfectly well be determined by catches 

 taken at intervals of a week and that the vertical distribution, 

 if computed from such observations, would agree very closely 

 with that reached from the very much larger number actually 

 used. Of course the larger and rarer forms, like Epischura and 

 Leptodora, vary in number very greatly. No one would at- 

 tempt to compute the population of a lake from the presence of 

 a single Leptodora in the catch, or from the occasional presence 

 of half a dozen, or more, but the numbers of the Crustacea which 

 are the regular constituents of the limnoplankton vary within 

 comparatively narrow limits in lake Mendota, and I feel confident 

 that my averages fairly represent the crustacean population. 

 The variation of the numbers of the Crustacea in lake Mendota 

 does not support extreme views either on the side of uniformity 

 of distribution or the opposing theory of swarms. 



In connection with reconnoisance observations it may be well 

 to remember the following : Exceptionally large catches are 

 due to the presence of great numbers of young, and exception- 

 ally small ones usually contain few young. A catch containing 

 great numbers of young may therefore be suspected to be un- 

 usually large and one with few youag, if taken in summer or 

 fall, to be small for the lake from which it comes. 



THE VERTICAL DISTRIBUTION OF THE CRUSTACEA. 



In making collections to determine the vertical distribution of 

 the Crustacea the same general method was followed as that de- 

 scribed in detail in my former paper. (Birge, '95, p. 429.) The 



