310 The Entomostraca of Mid-Lothian. [Sess. 
the surface of the water, and the eggs are hatched in the © 
following spring. We have occasionally found the water at 
the edge of a pond thickly covered with these floating 
ephippiums. Daphinias generally seem to die off in the 
winter, though we have on a few occasions found swarms of 
them even then. Baird, writing of the South of England, says 
that in a mild season they may be found all through the 
winter. 
In studying these little crustaceans we are confronted with 
the same perplexing question that arises with regard to higher 
organisms—“ What constitutes a species?” To take the daph- 
nias as an example, Brady recognizes as distinct species D. 
lacustris and D. galeata, while Lilljeborg includes both these 
forms under the name D. hyalina. Writing in 1898, Brady 
says, “ Professor G. O. Sars now reckons as mere varieties of 
D..longispina no fewer than eleven forms which had been 
previously described by himself or other authors as distinct 
species.” 
In the days before the theories of evolution and natural 
selection were familiar to students, the origin of this multi- 
plicity of closely related forms must have been extremely 
puzzling ; but we now know the daphnia to be a very variable 
organism, which responds readily to changes in its environ- 
ment; and we believe that the various forms met with in 
different localities are all modifications of one or two original 
species, slowly diverging more and more from each other, as 
each is gradually acted upon by its own peculiar surroundings. 
We are also in a position nowadays to conjecture with some 
degree of probability the significance of the peculiar characters 
found in certain young forms, since we know that in every 
young creature the life-history of its ancestors is more or less 
clearly indicated. Take, for example, the pointed head 
commonly found in young specimens of D. galeata ; this is a 
peculiarity characteristic of the adult Hyalodaphnia cucullata 
and certain other northern species, believed to be of pelagic 
origin; and suggests that the ancestors of D. galeata lived 
for generations under conditions similar to those now prevailing 
in the present home of those northern species, (These remarks 
apply to D. galeata, Sars, as described by Brady in “ Natural. 
History Transactions of Northumberland, Durham, and New- 
Sa ee Wis 
Ps 
