SUMMARY 217 



difference in composition of the two media. A con- 

 siderable number of marine organisms appear to be 

 endowed with the requisite adaptability, but not all : 

 further information is required on this point, and a wide 

 field of experiment here lies open. 



The second step is to acquire some means of defence 

 against extremes of climate, chiefly against cold in the 

 temperate regions, against drought in the tropics. This 

 is attained by the production of eggs or gemmules pro- 

 tected by resistent envelopes, or by the habit of 

 hibernation or aestivation. 



The third step is to obtain fixity of tenure and means 

 of dispersal. This has been anticipated already by the 

 formation of the protected eggs and gemmules, though 

 some trifling modifications have been added to fit these 

 for both purposes. 



There are some freshwater animals, like Dreissena, 

 which have not taken the third step ; their hold on the 

 freshwater is consequently precarious. 



An organism may take the first step while residing near 

 the mouth of a river, and may then enter the river basin 

 either by its own unaided exertions or as a passenger 

 carried by a fish, reptile, bird, or, in these days, by boat. 

 On the other hand, a whole world of marine organisms, 

 for no fault of their own, may become imprisoned in a 

 closed sea by movements of the earth's crust, and if this 

 should become filled with fresh water by the influx of 

 rivers, they are under compulsion to take the first step or 

 perish. Probably but few survive. 



A study of the earth's crust reveals to us the repeated 

 formation of lakes by the process just imagined, and 

 following the existence of these lakes, or even contem- 

 porary with them, a number of freshwater animals make 

 their appearance. 



And finally, a study of existing rivers discovers to us 



