22 



portion of tlie year ; that the matm^e shells are rarely found 

 on the shallow margins of lakes or pools, where collectors 

 are often obliged to obtain their specimens ; that the young 

 of various species differ very materially in ornament and 

 general form from mature specimens ; that such a remarkable 

 form as Gimdlachia petterdi (mihi), only 2 and 3 mil. long., 

 and being transparent, is not to be found, even where condi- 

 tions are favorable, without the aid of a keen experienced 

 eye ; and therefore, when we take all things into considera- 

 tion (not forgetting the paucity of experienced collectors), 

 we ought to be assured that the various districts have been 

 fully worked up before instituting final comparisons with 

 distant regions. Even then, the recent fossil deposits of 

 fresh water shells on Barren and Badger islands show how 

 important connecting links in time and space may easily be 

 overlooked or ignored. It must be remembered that conti- 

 guous, though completely isolated, pools may easily be stocked 

 with certain fresh water shells where the conditions are 

 favorable. 



That wonderful observer, Mr. Darwin — whose care and 

 patience are such that nothing important, however apparently 

 trivial, escapes his notice — writes : — " Some species of fresh 

 water shells have a very wide range, and allied species, which, 

 on my theory, are descended from a common parent, and 

 must have proceeded from a single source, prevail throughout 

 the world. Their distribution at first perj^lexed me much, 

 as their ova are not likely to be transported by birds, and 

 they are immediately killed by sea water, as are the adults. 

 I could not even understand how some natiu-alized sj^ecies 

 have rapidly spread throughout the same coimtry. But two 

 facts throw some light on the subject. When a duck sud- 

 denly emerges from a pond covered with duckweed, I have 

 twice seen these little plants adhering to its back ; and it has 

 happened to me, in removing a little duckweed from one 

 aquarium to another, that I have quite unintentionally 

 stocked the one with fresh water shells from the other. But 

 another agency is perhaps more effectual. I suspended a 

 duck's feet, which might represent those of a bird sleeping 

 in a natural j)ond, in an aquarium where many ova of fresh 

 water shells were hatching ; and I found that numbers 

 crawled on the feet, and clung to them so firmly that, when 

 taken out, they could not be jarred off. These just-hatched 

 molluscs, though aquatic, 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 hun- 

 dred miles, and would be sure to alight on a pool or rivulet 

 if blown across sea to an oceanic island, or to any other dis- 



