380 



SCIENCE. 



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



void of plant life. On the other hand in 

 shallow lakes not only do we find the float- 

 ing vegetation as in the deep lakes, but as 

 the light reaches the bottom over a larger 

 proportion of its surface, we have in addi- 

 tion a very large flora flourishing on the 

 bottom. In some of the very shallow lakes 

 nearly the whole bottom is covered with a 

 rank vegetation. This is true, for instance, 

 of Lake Vieux Desert. In Green Lake, on 

 the other hand, inasmuch as the shores are 

 somewhat precipitous, there is only a com- 

 paratively narrow margin on which can be 

 supported a flora growing upon the bot- 

 tom, while the larger part of the lake is so 

 deep that only the floating vegetation can 

 exist. It is easily seen, then, that a shal- 

 low lake will be ' plankton-rich ' as com- 

 pared with a deep lake. Fishermen recog- 

 nize this fact, and expect the shallow lakes 

 to be better for their sport. 



It is evident, then, that the living lim- 

 netic vegetation must be at or near the sur- 

 face, where it can have an abundance of 

 light. Animal life, however, is not limited 

 in this way. It was long ago shown that 

 in the sea there was an abundant surface 

 fauna and an abyssal fauna, but in regard 

 to the condition of the intermediate region 

 there has been some dispute. Agassiz has 

 claimed that there is a region intermediate 

 between the top and bottom, which is en- 

 tirely devoid of life. This has been dis- 

 puted by some authors, and late explora- 

 tions seem to indicate that no region be- 

 tween the surface and bottom is entirely 

 free from animals. A similar condition 

 exists in the lakes. By far the most abund- 

 ant fauna is at and near the surface, but 

 animals are found in greater or less num- 

 bers at all depths. The larger part of the 

 plankton is found within thirty or forty 

 feet of the surface ; but the same kinds of 

 animals that form the fauna of the upper 

 waters may be found at all depths, although 

 in small numbers. Limnocalanus is an ex- 



ample of an animal which belongs to the in- 

 termediate regions. It, too, may be found 

 in small numbers at any depth from the 

 surface down, but it seldoms occurs in any 

 considerable numbers outside the inter- 

 mediate region. 



Limnocalanus and Daphnia pulicaria are 

 perhaps the only animals in fresh water 

 which belong distinctively to the zonary 

 plankton, although Cyclops brevispinosus is 

 much more abundant between five and 

 twenty meters than it is near the surface. 



Collectors of plankton material have 

 known that they could ordinarily make 

 much more abundant collections at night 

 than in the daytime. This has led to a 

 belief that there is a vertical migration of 

 the plankton, towards the surface at night, 

 and away from it in the daytime. It was 

 supposed that the whole body of the plank- 

 ton moved up and down. This idea has 

 been proved to be false. What movement 

 there is is within quite nari-ow limits near 

 the surface, and all members of the limnetic 

 fauna do not, by any means, behave in the 

 same way. They have most decided in- 

 dividual peculiarities, so that we cannot 

 speak of the movements of the fauna as a 

 whole, but each species must be considered 

 by itself. Some of them do not move at all 

 vertically, but have the same distribution 

 from one end of the day to another. Oth- 

 ers, like the larval forms of the copepods, 

 are more numerous at the surface in the 

 daytime than in the night. Some have a 

 very pronounced migration. This is par- 

 ticularly true of Leptodora which is rarely 

 found at the surface in the daytime, but ap- 

 pears at almost exactly forty-five minutes 

 after sunset, remains at the surface dur- 

 ing the night, and disappears again at 

 just three-quarters of an hour before sun- 

 rise. 



Most of the larger Crustacea which form 

 the great body of the plankton do migrate 

 in this way, and it was natural, perhaps, to 



