L.M.B.C. TURBELLARIA. T5i 
form ”’ of the ice-age, has induced 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 Graubundtner Alps to the Maine Valley is the 
Rhine. 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 Rhine, 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. Thisis 
possible for fresh-water animals in two ways. (1) Winter 
egos or eges 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 
eges 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. 
