president's address. 
473 
to immigration nor even to water life in general, since the 
waters of the tropics are not more populous than those of 
temperate regions. Stability of temperature is a factor of 
some importance in colonisation of fresh waters, but it is quite 
subordinate to that of salinity. 
3. Currents. 
Sollas 1 has drawn special attention to the importance of 
currents in fresh water as causing a barrier to the immigration 
of marine animals. The majority of marine animals have 
small free-swimming larvae, and it is clear that a constantly 
descending current will wash out to sea all floating larval 
forms which cannot swim against it, so that no animal can 
become acclimatised which has not lost its free-swimming 
stages. It is certainly a fact that comparatively few fresh- 
water species have free-swimming larvae, though their nearest 
marine relatives have. For example, the Mollusc Anodonta 
sets free larvae which are peculiarly modified for attachment to 
fish and other animals by which they are distributed. The 
crayfish, again, hatches its young in a condition to adopt the 
parent’s habit of life, while the young lobster spends some 
weeks in a comparatively helpless floating stage. But there 
are notable exceptions. The Mollusc Dreissensia, which is 
essentially a river form, has as simple a larva as any marine 
genus. Again, the larva of Palaemontes is even a less powerful 
swimmer than the larva of the lobster, and the estuarine 
Cordylophora has a ciliated larva. It seems that the river 
current is not an effective barrier against marine animals, so 
far at least as concerns the tidal parts, which alone can be in 
question. For example, the whole volume of water at the 
mouth of a river will flow up the river for about ten miles 
during the flood tide. It will then flow back again for about 
twelve miles, and so end the twelve hours two miles out to sea. 
It is obviously easy for a floating larva at the stage of final 
metamorphosis to settle anywhere within that ten miles. 
But, unless such a larva can complete its metamorphosis 
within the period of a complete tide, it would be impossible for 
1. Sollas. Trans. Roy. Soc. Dublin. (2) III., 1884. 
