Gatty Marine Laboratory , St. Andrews. 157 



those wlio arc 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 arc crushed by the Scari, or to the sedentary Poly- 

 chiets 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 Filoyrana, or in that of 

 Obelia, and it can hardly be said that there is a struggle for 

 existence in such reef -corals as Palytlioa or Zounthus. 



Checks there must l)e on the extraordinary powers of 

 propagation shown by Fi/o_(/;rt7ia, else the ocean would swarm 

 with masses like 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 " })erforming 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 meinbrunous, 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 branchiai 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 oedijicatrix are characteristic of the 



