752 FRESH-WATER BIOLOGY 



Mention has been made of the fact that new species may arise 

 because of isolation. But the question arises, how do the ancestors 

 of any form first reach a given body of water? By what means 

 are these animals distributed from one place to another? Certain 

 species occur in bodies of water from one side of the continent to 

 the other; in some cases the same forms are found even in widely 

 separated continents. How have they reached these places? Eggs 

 are formed which fall into the mud of the floor of the lakes and 

 pools and retain their vitality, sometimes from one season to 

 another, even if the bodies of water disappear and the mud becomes 

 dry. Many species have been seen for the first time by rearing 

 them from eggs found in dried mud. It is natural to infer from 

 this that anything that would move the mud would also move the 

 eggs of the animals. Dried mud, in the form of dust, may be 

 widely disseminated, and thus the eggs might be carried to very 

 distant places. Water birds, too, carry mud on their feet from 

 one body of water to another, and in this way may easily transport 

 the eggs of Copepoda and possibly the living animals. Inasmuch 

 as these birds sometimes make long flights, it is clear that the 

 species of Copepoda may be planted in places far apart. There is 

 no doubt that in both these ways the distribution of the Copepoda 

 has been effected, but these are not the principal ways. It seems 

 evident, for many reasons, that they go from one place to another 

 mainly by direct water carriage. For example, Diaptomus sici- 

 loides has been found in only one lake in Wisconsin. If it were 

 readily carried by birds, one would expect to find it in other bodies 

 of water which seem to have the same kind of an environment. 

 On the other hand, in Lake St. Clair, although it is a very shallow 

 body of water, occur the Copepoda that are characteristic of the 

 deeper waters of the Great Lakes. In this case there seems to be 

 no doubt that these deep-lake forms have been carried into an en- 

 vironment where one would not expect to find them. It is notice- 

 able that in connecting bodies of water we find the same forms of 

 Copepoda. Irrigating ditches and ponds are almost entirely with- 

 out Copepoda. They are peculiarly unprofitable collecting places 

 although the environment would seem to be favorable for the propa- 

 gation of these forms. For some reason, it is evident that the 



