384 



SCIENCE. 



[N. S. Vol. XI. No. 271. 



The question of the uniformity of hori- 

 zontal distribution has been discussed by 

 various authors and with considerable 

 vigor, but I think it must now be acknowl- 

 edged, that while there is a certain amount 

 of uniformity, bo that, by a considerable 

 number of collections, we can express 

 within rather wide limits the amount of 

 plankton at any time on a lake, uniformity 

 in any exact sense does not exist. This 

 lack of uniformity is largely due to a dif- 

 ference in the number of Crustacea, al- 

 though there are marked differences in the 

 distribution of the other organisms. Sur- 

 face growing plants, for instance, are moved 

 about under the influence of the winds, and 

 accumulate on the leeward side of a lake. 

 If one part of a lake is deeper than an- 

 other and the lake is not much disturbed 

 by the winds, at the period of maximum 

 growth the number of diatoms will be much 

 greater over the deeper part. Crustacea 

 may be in ill-defined aggregations which 

 may be called swarms, and these swarms 

 are not stationary in all cases, but move 

 slowly, perhaps under the influence of ex- 

 tremely weak currents. Not only is there 

 a considerable variation in collections made 

 at different locations on a lake at the same 

 time, but if a series of collections is made 

 at the same place, the amount of the 

 plankton in some collections may be twice 

 as much as in others. An examination 

 will show that these large variations are 

 generally due to a difference in the num- 

 bers of some of the Crustacea, showing con- 

 clusively that not only do the numbers of 

 Crustacea vary at different locations, but that 

 these swarms are not stationary. It fol- 

 lows, then, that conclusions in regard to the 

 plankton drawn from a few collections may 

 be quite erroneous. If, however, a consid- 

 erable number of collections is made, especi- 

 ally if they are made from widely-separated 

 localities, the average of all these collec- 

 tions, allowing something of a margin for 



error, will give a fair idea of the amount 

 of plankton in a lake. Of course, the 

 larger the number of collections the less 

 the amount of error, but anything like very 

 exacL results can not be expected. We are 

 able, in this way, to compare the plankton 

 of one lake with that of another, or to de- 

 termine the relative amount of plankton at 

 different times of the year on any single 

 lake. But any estimate of the actual 

 amount of plankton produced by a lake at 

 any time or during the year must be ac- 

 knowledged to be only an approximation. 

 Care must be taken, too, in comparing one 

 lake with another, that they be compared 

 under similar conditions. The maximum 

 of one lake may not be reached at the same 

 period as that of another. The conditions 

 of a deep lake are very different from those 

 of a shallow lake, and a fair comparison 

 can be made only by averaging collections 

 continuing over a considerable period. 



In the abyssal region, because of the lack 

 of light, plant life is impossible, and the 

 fauna is very meager. 



It is true that the list of animals which 

 have been found in the abyssal regions of 

 lakes is a long one, including, as it does, pro- 

 tozoa, ccelenterata, worms, molluscs, bry- 

 ozoa, Crustacea, arachnida, insect larvae, and 

 some few fish, but an examination of any 

 single lake shows that not only is the num- 

 ber of kinds small, but the numbers of in- 

 dividuals of any kind are very small. In 

 Europe the abyssal fauna of Lake Leman 

 has been worked up with great thorough- 

 ness. In this country very little detailed 

 work has been done on this subject. It is 

 not a fruitful field for research, and it is not 

 strange that it has been neglected. In 

 shallow lakes it is doubtful if there is any 

 distinctive abyssal fauna. The most abun- 

 dant animals in the mud of the bottom are 

 worms, insect larvae, gasteropod and lamelli- 

 branch molluscs, and amphipods. With 

 these may be associated at times great 



