L.M.B.C. TURBELLARIA. 57 



form" of the ice-age, has mduced von Kennel to oppose 

 this easy method of solving the problem and to try 

 whether recent migration of an active or passive nature 

 may not sufficiently account for the facts. In the first plfice 

 it is clear that (assuming recent migration) although active 

 wandering may account for the occurrence of Planaria 

 alpina in the Alps and in southern Germany, it cannot 

 apply to England since, by hypothesis, England was sep- 

 arated by sea from the Continent. The only water-way 

 from the Graubiindtner Alps to the Maine Valley is the 

 Khine. Now although the water in the upper part of this 

 river would be of a sufficiently low temperature to allow 

 of alpina living in it, lower down the temperature is 

 too high. If however, gradually from time to time, 

 chiefly in the winter, migration had gone on extending 

 further and further down the Ehine, suitable places might 

 have been secured in the Maine Valley where the species 

 might still be found. Such occurrences would scarcely 

 have escaped the notice of the Germans, and as I said 

 above, this reasoning cannot explain the presence of this 

 form in countries separated by arms of the sea. 



Next then, let us consider a passive migration. This is 

 possible for fresh-w^ater animals in two ways. (1) Winter 

 eggs or eggs with resistant membranes might be carried 

 by the wind to distant regions and (2) eggs or young may 

 transferred by fish or birds, &c. Planaria alpina lays its 

 eggs in capsules possessing chitinous walls. These 

 capsules are attached to leaves or stones, and might be 

 carried by the agencies I have mentioned. If either 

 method were used we might expect to find this creature 

 more generally distributed than is the case. It must 

 however be remembered that it is only in certain places 

 that the conditions are sufficiently favourable for exis- 

 tence, von Kennel gives an interesting example of this. 



