254 



SCIENCE. 



[N. S. Vol. XI. No. 268. 



interpretation of these results more knowl- 

 edge is necessary. I shall content myself 

 with pointing out a few directions in which 

 this interpretative knowledge must be 

 gained, if the results of observation are to 

 be of real scientific value. First in im- 

 portance I should place the knowledge of 

 the chemistry of the plankton, from which 

 its possible nutritive value can be learned. 

 The unit system of counting, as advocated 

 by Whipple, is undoubtedly a great advance 

 on mere enumeration, but the units thus 

 employed are not even units of mass, much 

 less units of chemical composition. The 

 difference in the amount of ash between 

 diatoms and Crustacea, not to speak of 

 other differences, is so great that no direct 

 comparison, however exact, of cubic con- 

 tents can teach us the significance of the 

 two groups in the lake's plankton. Chem- 

 ical analyses only will do this and these 

 must be be stated both in terms of bulk of 

 the centrifuged or settled plankton, and in 

 terms of the average individual plant or 

 animal, as determined by counting. Every 

 limnologist, therefore, should be careful to 

 avail himself of the opportunity presented 

 by the appearance of a monotypic plankton 

 to collect it in sufficient quantities for 

 analysis. I have myself to confess that I 

 have neglected three opportunities of col- 

 lecting Daphnia hyalina in these large quan- 

 tities. Similar opportunities have no doubt 

 been neglected by many of us, but the 

 necessity of this information is so pressing 

 that I am sure we must all agree in placing 

 it first on the list of desiderata. 



It may be possible to collect the ordinary 

 plankton in large quantities and to separate 

 it into its constituents more or less com- 

 pletely by means of one of the centrifugal 

 milk separators. The difference in density 

 between the diatoms, the cyanophycese and 

 the Crustacea is so great that there ought 

 not to be any serious difficulty in separating 

 the plankton into at least three groups. 



So far as I am aware, no experiments have 

 been made in this direction, but the work 

 must be attempted and in some way or other 

 we must secure single groups, or, if possible, 

 single species of plankton plants or animals 

 in quantities sufficient for chemical analysis, 

 or our results will continue to suffer from 

 their present indefiniteness. 



A second line of investigation which de- 

 mands much study concerns the biological 

 significance of the constituents of the plank- 

 ton, especially of the plants. Many ob- 

 servers have noted that certain algse are 

 eaten more freely by the Crustacea than are 

 others, yet no such careful and continued ob- 

 servations have been made as to enable us 

 to make any general statements on the sub- 

 ject. Evidently, however, this work must 

 be done, or even the chemical knowledge 

 gained by analysis will fail to disclose tlie 

 real interrelations of the plankton plants 

 and animals. 



A third point of equally great importance 

 concerns our knowledge of the chemistry of 

 the water of the lakes and of its gaseous 

 contents as related to the plankton. Many 

 waters have been analyzed, yet few or no 

 attempts have been made to correlate these 

 analyses with the nature and abundance of 

 the several plankton species. Still more con- 

 spicuously is this true of gas analyses. We 

 suppose, for instance, that in certain lakes 

 the accumulation of the products of decom- 

 position in the deeper waters prevents the 

 animals of the upper regions from descend- 

 ing into the cooler waters of the lake, yet 

 we are at present entirely ignorant of the 

 nature of these products, whether they are 

 gaseous or other, and of the way in which 

 they are able to affect so powerfully the bio- 

 logical conditions of the sub-thermocline. 

 Numerous similar questions are pressing for 

 solution. 



A fourth class of questions comprises 

 those raised by the relation of the littoral 

 area to the limnetic region of the lake. 



