84 The Lake as a Microcosm. 
peculiar, but all were of the kinds found in the smaller lakes, and all occurred 
also in shallower water. It is evident that these interior regions of the lakes 
must be as destitute of fishes as they are of plants and lower animals. 
While none of the deep-water animals of the Great Lakes were found in 
Geneva Lake, other evidences of zoélogical affinity were detected. The tow- 
ing net yielded almost precisely the assemblage of species of Entomostraca 
found in Lake Michigan, including many specimens of Limnocolonus macru- 
rus, Sars.; and peculiar long, smooth leeches, common in Lake Michigan, but 
not occurring in the small Illinois lakes, were also found in Geneva. Many 
Valvata 3-carinata lacked the middle carina, as in Long Lake and other isolated 
lakes of this region. 
Comparing the Daphnias of Lake Michigan with those of Geneva Lake, 
Wis. (nine miles long and twenty-three fathoms in depth), those of Long Lake, 
Ill. (one and a half miles long and six fathoms deep), and those of other still 
smaller lakes of that region, and the swamps and smaller ponds as well, we 
shall be struck by the inferior development of the Entomostraca of the larger 
bodies of water, in numbers, in size and robustness, and in reproductive pow- 
er. Their smaller numbers and size are doubtless due to the relative scarcity 
of food. The system of aquatic animal life rests essentially upon the vegeta- 
ble world, although perhaps less strictly than does the terrestrial system; and 
in a large and deep lake vegetation is much less abundant than in a narrower 
and shallower one, not only relatively to the amount of water but also to the 
area of the bottom. From this deficiency of plant life results a deficiency of 
food for Entomostraca, whether of Algze; of Protozoa or of higher forms, and 
hence, of course, a smaller number of the Entomostraca themselves, with more 
slender bodies suitable for more rapid locomotion and wider range. 
The difference of reproductive energy, as shown by the much smaller egg- 
masses borne by the species of the larger lakes depends upon the vastly greater 
destruction to which the paludinal crustacea are subjected. Many of the lat- 
ter occupy wateos liable to be exhansted by drought, with a consequent enor- 
mous waste of Entomostracan life. The opportunity for reproduction is here 
greatly limited —in some situations to early spring alone — and the chances for 
destruction of the summer eggs in the dry and often dusty soil are so numerous ~ 
that only the most prolific species can maintain themselves under such condi- 
tions. 
Further, the marshes and shallower lakes are the favorite breeding grounds 
of fishes, which migrate to them in spawning time, if possible, and it is from 
the Entomostraca found here that most young fishes get their earliest food 
supplies —a danger from which power the deep-water species are measurably 
free. Notonly isa high reproductive therefore rendered unnecessary among the 
latter by their freedom from many dangers to which the shallow-water species 
are exposed, but in view of the relatively small amount: of food available for 
them, a high rate of multiplication would be a positive injury, and could 
result only in wholesale starvation. 
All these lakes of Illinois and Wisconsin, together with the much larger 
Lake Mendota at Madison (in which also I have done much work with dredge, 
trawl, and seine), differ in one notable particular both from Lake Michigan 
and from the larger lakes of Europe. In the latter, the bottoms in the deeper 
parts yield a peculiar assemblage of animal forms, which range but rarely into 
the littoral region, while in our inland lakes no such deep water fauna occurs, 
with the exception of the cisco and the large red Chironomus larva. At Grand 
Traverse Bay, in Lake Michigan, I found at a depth of one hundred fathoms 
a very odd fish of the sculpin family ( 7’riglopsis thompsoni, Gir.), which, until 
I collected it, had been known only from the stomachs of fishes; and there 
also was an abundant crustacean, Mysis,—the ‘opossum shrimp”’, as it is 
sometimes called — the principal food of these deep lake sculpins. Two re-— 
