90 SoLtas— On the Origin of Freshwater Faunas. 
So far from this being the case, we find that the freshwater forms of Mollusca 
are remarkably well defined from the marine, and that they maintain their dis- 
tinctive generic characters throughout a distribution generally world-wide in 
space, and extending far back into the Mesozoic period in time. 
Some other efficient cause or causes must then be sought for in this inquiry, 
and one—and that a most important one—lies obviously on the surface. Perhaps 
one of the commonest ways by which marine animals obtain a distribution over 
extensive areas is by means of free-swimming larve. The peopling of the sea by 
slow-moving or attached forms has certainly been accomplished chiefly, if not 
almost wholly, in this manner. But obviously no new forms can have been intro- 
duced into existing river-systems through the agency of free-swimming larvee, for 
these fragile and feeble forms can by no means make headway against the seaward 
current of a river: indeed, as a matter of observed fact, larvee are never known to 
swim against any current, but always along with it. And thus the method which 
has been most potent in disseminating organisms through the sea must have been 
wholly imoperative in transferring them to a freshwater habitat.* 
Furthermore, if any slow-moving animal had managed, in the adult state, to 
penetrate some little distance up a stream, it could seldom succeed in permanently 
establishing itself so long as it passed through a free larval stage; for its larvee 
would usually be carried away to the sea, where they would perish or resume 
the ancestral habits. Swift-moving animals, such as fish, would of course be more 
advantageously situated, since they could rapidly travel a long way up most 
streams, and might easily find at length some sheltered recess or quiet lagoon, 
wherein their young could come to maturity. The necessity for some such quiet 
spot is sufficiently indicated by the long and arduous journies which the freshwater 
Salmonidz take to reach it. Excepting fish (and we only propose to discuss the 
Invertebrates in this Paper) and minute organisms, such as Protozoa, minute 
Crustacea, Rotifera, and Tardigrades, which are capable of transport in a dessicated 
* This has been hinted at by Semper, as I found subsequently ; his words are :—‘‘I have already 
indicated that very often the strength of the current in a river, or the surf at its mouth, its 
temperature, or the kind of food it affords, must cause quite as great a hindrance to the passage of a 
marine animal into the fresh water as the necessity for subsequently living in water devoid of salt. 
Thus, for instance, the remarkably tender bodies of the larva of the Echinodermata, Ascidia, Sea- 
anemones, Hydroid polypes, and others, are scarcely fitted to overcome such impediments; so that 
even under the assumption that they might be capable of living in water without salt, their transfer 
into fresh water seems to be almost impossible ; and this is still more probably the case when the fully 
grown creatures—such as Ascidians, Corals, Polyps, and others—do not move freely on the sea bottom, 
but are permanently attached to it” (Animal Life, 1881, p. 149). I was quite under the impression 
that this idea was my exclusive property, and I have previously made use of it in explaining the 
exclusion of marine forms of Spongie from our rivers (Cassell’s Natural History, article ‘‘ Spongie,”’ 
p- 828, Part txx., 1882). But with such a mine of facts and wealth of ideas as occur in Semper’s 
work, how shall one be sure that any idea on this subject is one’s own ? 
