378 



SCIENCE. 



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



no plants are found. The study of the pelagic 

 and abyssal faunas has been enth-ely within 

 the last half centurj' ; in fact the very 

 knowledge of their existence dates back 

 hardly fifty years. 



In the lakes we use a similar set of terms. 

 The regions are ' littoral,' ' limnetic ' and 

 ' abyssal.' The characteristics of these re- 

 gions are somewhat known, but still our 

 knowledge is far from perfect. 



A list of the plants and animals found in 

 any one lake seems quite formidable be- 

 cause of its length, but the species that are 

 present in any considerable numbers are 

 very few. From a limnetic collection, for 

 instance, we may find in abundance the 

 following : Four or five copepods, five or 

 six cladocera, three or four protozoa, and 

 perhaps two rotifers. This would be a fair 

 average fauna in one of our lakes. Of the 

 plants, we would find two or three diatoms 

 and as many algse. 



Not only are the animals and plants of 

 neighboring lakes verj^ much alike, but the 

 same animals may be found distributed over 

 wide territories, and even over different con- 

 tinents. This is true even of some of the 

 higher animals of the plankton, like the 

 Crustacea. Of our fifteen species of Cy- 

 clops nine are found in Europe. In the case 

 of one species it is not only found in Europe, 

 but in Asia and in Africa, and literally does 

 not vary a hair in these widely separated 

 localities. On the other hand, the genus 

 Dlaptomus is very variable. We have not a 

 single species which is common to the Euro- 

 pean lakes. Not only that, but there are 

 many localized species in the United States. 

 One species occurs, so far as known, only 

 in a few lakes in the northern part of the 

 lower peninsula of Michigan. A second is 

 widely distributed in all the smaller lakes 

 across the continent in the northern States. 

 Another goes from the center of Wisconsin 

 north into the Arctic regions. In the Rocky 

 mountain regions are several peculiar spe- 



cies. Through the southern States two spe- 

 cies are found which never come north. 

 Mexico has at least one peculiar species. Of 

 the other organisms, both animal and plant, 

 most are world-wide in their distribution. 

 From this fact of the general uniformity of 

 fauna and flora over wide regions, it is clear 

 that the study of a lake which simply pro- 

 duces a faunal and floral list is of very little 

 value. There was a time when such lists 

 were important, before this uniformity of 

 distribution was determined, but that time 

 is long since past, and those European 

 authors who continue to fill the proceedings 

 of learned societies with lists resulting from 

 desultory explorations of one or more lakes 

 are almost wasting printers' ink. 



In the littoral region we find usually an 

 abundant flora. Those plants which need 

 an anchorage find it in the mud of the 

 bottom, and the unattached plants are pro- 

 tected b}' those that are stationary. Pro- 

 tected by these plants and living upon 

 them is an abundant fauna in which Crus- 

 tacea are the most prominent, although we 

 find great numbers of rhizopods, infusoria, 

 sponges, hydrozoa, worms, true insects and 

 mites, mollusca and bryozoa. This abund- 

 ance of the lower animals forms a rich 

 supply of food upon which the higher ani- 

 mals can live. It is in this littoral region 

 that the fish get the most of their food, and 

 every fisherman knows that marshy borders 

 are necessary to maintain the supply of 

 fish. The animals of the littoral plankton 

 are opaque, and generally are so colored 

 that when they are at rest they are incon- 

 spicuous. Those that swim about and then 

 drop to the bottom to rest are ordinarily so 

 colored that they are not easilj' distinguished 

 from the mud. Frequently in littoral regions 

 the bottom is covered with a thick mat of 

 Chara, which, in turn, forms hiding places 

 for enoi-mous numbers of the invertebrates. 



The flora of the limnetic region can be, 

 of course, only of floating plants. Among 



