212 Transactions. — Zoology. 



well-stocked creeks and drains, has now numbers of the 

 Pisidiwn in it. 



Now, the question arises, How did these slow-moving, 

 water-loving shells manage to so thoroughly populate these 

 isolated outlying lakes and pools ? One can understand their 

 presence up all the streams : they are free-swimming for a 

 time after leaving the egg, thus enabling them to disperse 

 rapidly over the stiller waters, though this would act rather 

 as a disadvantage than otherwise in our rapid streams — the 

 feeble, tiny folk must be swept out to sea in millions during 

 floods ; more is due to the slow crawling of the adult shells, 

 working through the ages up stream. 



But, though both pond- and river-snails will pass up or 

 down the tiniest of runlets, or even over moist ground for 

 short distances, the tracts of dry land between many of our 

 pools and the nearest running water are far too wide for their 

 crossing unaided. As for the river mussel and cockle, their 

 powers of locomotion are even more limited. 



It was not without purpose that I mentioned our waters 

 as the refuge for hunted water-fowl. Let me quote a cele- 

 brated experiment of Mr. Darwin's : "I suspended the feet of 

 a duck in an aquarium where many ova of fresh-water shells 

 were hatching, and I found that numbers of the extremely 

 minute and just-hatched shells crawled on the feet, and clang 

 to them so firmly that, when taken out, they could not be 

 jarred off, though at a somewhat more advanced age they 

 would drop off. These just-hatched molluscs, though aquatic 

 in their nature, survived on the duck's feet in damp air from 

 twelve to twenty hours ; and in this length of time a duck or 

 heron might fly at least six or seven hundred miles, and, if 

 blown across the sea to an oceanic island, or to any other 

 distant point, would be sure to alight in a pond or rivulet." 

 Now, just watch a wild duck as, with startled resonant 

 " quack," he rises f rom the water or oozy shore of his- 

 feeding- grounds, how neatly the folded web-feet are tucked 

 under his tail as he hurries off inland for, perhaps, a thirty- 

 mile flight before resting the sole of his foot on ground again. 

 Look round any of our lake-edges and still streams, and note 

 what hosts of tiny shells there are in the ooze of the margin ; 

 then imagine the broad, lined surface of the duck's foot 

 strewn with these shells, as, with restful, satisfied splash, 

 he alights in some quiet upland pool, to tenant it unwittingly 

 with tiny shells. 



On a visit to the Flagstaff waterholes — small lakes that, 

 as I have said before, swarm with mussels — searching in the 

 mud of the margin, I found young river snails and cockles 

 very abundant, pond-snails decidedly scarce, and of mussel- 

 fry none at all, only securing specimens of the last with the 



