Gatty Marine Laboratory, St. Andrews. Eee 
those who are best able to obtain food regularly, and avoid 
their numerous enemies.” It is difficult, however, to see 
how such an argument can apply to sedentary zoophytes 
which are browsed on by young cod, to the living corals 
which are crushed by the Scari, or to the sedentary Poly- 
cheets in calcareous tubes which are devoured by Echini 
and various fishes. It would be interesting to find out in 
these the “ struggle for existence in which the weakest and 
least perfectly organised must always succumb.” There is 
little competition in a colony of Filograna, or in that of 
Obelia, and it can hardly be said that there is a struggle for 
existence in such reef-corals as Polythoa or Zoanthus. 
Checks there must be on the extraordinary powers of 
propagation shown by Filograna, else the ocean would swarm 
with masses lke coral-reefs, yet individual competition 
‘must be slight, since post-larval forms secrete their tubes, it 
may be, on new sites, whilst the buds may increase the 
parent mass of tubes on the old one. Each is perfect and 
capable of ‘‘ performing the different acts necessary to its 
safety and existence under all the varying circumstances by 
which it is surrounded,” and “ perfect acquaintance with its 
organization and habits” would hardly enable us ‘to 
calculate the proportionate abundance of individuals which 
is the necessary result.” It cannot be said that the inhabi- 
tants of the sea are “ kept down by a periodical deficiency 
of food,” though other checks exist. It is difficult also to 
explain the comparative abundance, say, of Filograna or 
the scarcity, say, of Placostegus as due to their organization 
and resulting habits, “which, rendering it more difficult 
to procure a regular supply of food and to provide for their 
personal safety in some cases than in others, can only be 
balanced by a difference in the population which have to 
exist In a given area.”’ 
If it be supposed that the ancestral form was devoid of an 
operculum, and that the presence of that organ in one form 
or another is a variation, the question as to its influence on 
the welfare of the species naturally suggests itself. Can the 
thin, almost membranous, operculum so guard the aperture 
of the tube as to be a decided advantage to the occupant—in 
contrast with the bare tips of the branchiz or their enlarged 
extremities, which otherwise block it? The indiscriminate 
occurrence, in the same colony, of opercula, enlarged tips, 
and ordinary tips, would point to the view that the develop- 
ment of one or other of these is of secondary moment ; yet 
it must be borne in mind that in certain northern localities 
the majority follow one condition or another, and that 
such races as Salmacina cedijicatrix are characteristic of the 
