54 THE BIOLOGY OF THE SEA-SHORE 



carried into unfavourable environments will succumb, while 

 those that are more fortunate in this respect are still faced 

 with the problem of obtaining a foothold, since in favourable 

 localities almost every possible spot may be already pre- 

 empted. These factors may prevent the appearance of 

 certain species in many localities, but should even small 

 changes occur in a locality unfavourable to some form or 

 forms, immigrants may at once appear. Given a foothold, 

 a further struggle ensues, namely, that for maintenance of 

 life in competition with the local species ; while yet a final 

 stage exists, in which reproduction and the fixation of the 

 young must again be accomplished before the species can 

 persist as a regular member of the community. Occa- 

 sionally a new-comer may be better fitted for the struggle 

 than some local species, and will then more or less rapidly 

 increase its numbers and range until checked by encounter- 

 ing limiting factors once more. 



Introduction of New Forms. — The following are instances 

 illustrating the above : 



Among the algae, Colpomenia sinuata appeared suddenly 

 on the coasts of Brittany and Cornwall, and in the course 

 of a few years became completely naturalised over a large 

 area on both sides of the English Channel (Cotton, op. cit.). 

 Hedley {op. cit.) records that in Sydney Harbour this species 

 is likewise active, and says that " occasionally the Zosteretum 

 is invaded by the Bubble-weed C. sinuata (see Stead, 

 1 91 2). This pest spreads over the flats, smothering other 

 vegetation and attaching itself to the oysters, whelks, or 

 cockles. When the gas forms in the expanding balloon it 

 lifts the shell and floats it away, so that oyster plantations 

 are sometimes seriously denuded by its agency." The same 

 writer notes that certain species periodically colonise within 

 the area (Sydney), being brought by the Notonectian flood 

 from the tropics ; Bonellia, for instance, then gains a footing, 

 but perishes when the flood swings offshore ; Stromhus 

 luhuanus, common and conspicuous on coral reefs, was 

 abundant on some rocks, but disappeared from May. 1865, 

 to April, 1896, when it reappeared. 



