DISPERSAL OF SLUGS. 1 75 



either with or without a slime-thread, from small 

 objects upon which they may be placed, and young 

 slugs, it should not be overlooked, soon perish from 

 exposure, so that after a little time, even if they ad- 

 hered, they would be sure to dry up and die. Dr. 

 Scharff raises a further difficulty ; supposing that a 

 couple of young slugs, he writes, happened to stick to 

 the feet of a bird and were carried a long distance 

 without being shaken or blown off — which in itself he 

 regards as unlikely enough — on being safely deposited 

 on the ground they would " still have to keep together 

 until mature, and not lose sight of one another in order 

 to propagate their species." The difficulties constantly 

 attending the establishment of new colonies as the result 

 of all occasional means of dispersal, it must of course be 

 admitted, are very great, and this fact, as already 

 insisted, must never be lost sight of, but in connection 

 with the present point it may be useful to remember 

 that many kinds of birds migrate in immense numbers 

 year after year along the same lines. To give an 

 imaginary case : hundreds of migratory birds, let us 

 suppose, roost together among tufts of herbage in a 

 swamp, where slugs, hatching out from the egg, and 

 abounding on al! sides, adhere in numbers to their feet ; 

 proceeding on migration, after a few hours, over an arm of 

 the sea, they might deposit on the opposite shore a number 

 of individuals, which, joining others similarly transported 

 in former years, might become firmly established in the 

 new home ; or losing their way in a fog or otherwise, 

 the birds might chance to alight altogether on some 



