322 OUTLINES OF EVOLUTIONARY BIOLOGY 



dispersal even of large and active fish, like the mackerel, is largely 

 assisted by the action of currents upon the floating eggs, and 

 this factor must be of still greater importance in the case of 

 the comparatively sluggish bottom dwellers, such as the turbot 

 and sole. 



The more superficial waters of the open ocean are densely 

 populated with pelagic animals and plants and with pelagic eggs 

 and larvae in various stages of development, all drifting more or 

 less helplessly wherever the ocean currents may carry them, for 

 their own powers of locomotion are usually quite insufficient to 



enable them to pursue an inde- 

 pendent course. This floating 

 population is technically spoken 

 of as " plankton " and its in- 

 vestigation, which is of great 

 importance for the solution of 

 practical fishery problems, has 

 lately attracted a great deal of 

 attention. 



We must therefore regard all 

 the great ocean currents, such 

 as the Gulf Stream, as highways 

 thronged with life of many kinds, 

 including representatives of all 

 the more important groups of 

 marine animals, any one of 

 which may be on its way to 

 found a new colony and establish its own particular species in some 

 region far distant from its original home. Some of the wanderers 

 are only immature forms, belonging partly to shore-dwelling 

 species, others are adult. Almost all exhibit some special adap- 

 tation to their pelagic mode of life. The fish-eggs are floated 

 by oil-globules, and the larvae of the crabs, brittle stars and 

 sea-urchins are provided with defensive spines (Figs. 127, 128) ; 

 but the most general and characteristic feature of all the pelagic 

 host is transparency, whereby they are rendered inconspicuous 

 and less likely to become the victims of the numerous enemies 

 which feed upon the plankton. Adult jelly-fish, worms, molluscs 

 and crustaceans, and innumerable larval forms, all exhibit this 

 same peculiarity. 



The effect of ocean currents upon the distribution of marine 



FIG. 164. Free-swimming Larva of 

 a Sponge, Orantia compressa ; 

 highly magnified. 



(The larva swims by means of the rapid 

 undulations of the numerous flagella 

 with which it is provided.) 



