h sides of the North Atlantic,
and to the South African and Chinese seas.

_Confined range of other species._--Mr. Lowe, in a memoir published in
the Cambridge Transactions in 1834, enumerates seventy-one species of
land Mollusca, collected by him in the islands of Madeira and Porto
Santo, sixty of which belonged to the genus Helix alone, including as
sub-genera Bulimus and Achatina, and excluding Vitrina and Clausilia;
forty-four of these are new. It is remarkable that very few of the
above-mentioned species are common to the neighbouring archipelago of
the Canaries; but it is a still more striking fact, that of the sixty
species of the three genera above mentioned, thirty-one are natives of
Porto Santo; whereas, in Madeira, which contains ten times the
superficies, were found but twenty-nine. Of these only four were common
to the two islands, which are separated by a distance of only twelve
leagues; and two even of these four (namely _Helix rhodostoma_ and _H.
ventrosa_) are species of general diffusion, common to Madeira, the
Canaries, and the south of Europe.[919]

The confined range of these mollusks may easily be explained, if we
admit that species have only one birth-place; and the only problem to be
solved would relate to the exceptions--to account for the dissemination
of some species throughout several islands, and the European continent.
May not the eggs, when washed into the sea by the undermining of cliffs,
or blown by a storm from the land, float uninjured to a distant shore?

_Their mode of diffusion._--Notwithstanding the proverbially slow motion
of snails and mollusks in general, and although many aquatic species
adhere constantly to the same rock for their whole lives, they are by no
means destitute of provision for disseminating themselves rapidly over a
wide area. "Some Mollusca," says Professor E. Forbes, "migrate in their
larva state, for all of them undergo a metamorphosis either in the egg
or out of the egg. The gasteropoda commence life under the form of a
small spiral shell, and an animal furnished with ciliated wings, or
lobes, like a pteropod, by means of which it can swim freely, and in
this form can migrate with ease through the sea."[920]

We are accustomed to associate in our minds the idea of the greatest
locomotive powers with the most mature and perfect state of each species
of invertebrate animal, especially when they undergo a series of
transformations; but in all the Mollusca the