in] THE FACTORS OF DISTRIBUTION 65 



opportunity of feeding and being drifted about by 

 the tides, winds and currents over as wide an area as 

 possible. After a pelagic phase of variable duration 

 the larva undergoes one or more metamorphoses and 

 assumes the form of the parent, and then settles 

 down on the sea-bottom. The larva is usually loco- 

 motory though the adult is not : thus the rooted 

 immoveable algae produce motile zoospores and the 

 fixed zoophytes free-swimming medusoid larvae. 

 But whether or not the larva is locomotory it is so 

 small, and its specific gravity is so near that of sea- 

 water, that it is carried for great distances by tidal 

 streams and currents, and it may settle down very far 

 away from the place where it was born. Its life as 

 a larva is usually a very limited one if the free- 

 swimming stage lasted long enough there would not 

 appear to be any limit to the normal distribution of 

 the species to which it belongs. The object of the 

 development of the larval stage if we may speak of 

 an ' object ' in an evolutionary process is obviously 

 the enlargement of the area of distribution of the 

 species. 



This distribution of a sessile, or semi-sedentary, 

 species by means of free-swimming larvae must be, 

 to some extent, a fortuitous process. For though 

 the larvae may appear to be very active creatures 

 when examined in a few drops of water under a 

 microscope, yet their powers of locomotion are very 



J. 5 



