in] THE FACTORS OF DISTRIBUTION 79 



until it can no longer be found. The area within the 

 contour line we may call the area of productive 

 distribution, that without it, and including all parts 

 of the sea where the species has occurred, we may call 

 the area of nominal distribution. It is unable to 

 maintain itself within the latter area unless it is 

 recruited from the former one. 



The individuals of a species may be carried far 

 beyond their area of optimal distribution, for if pelagic 

 larvae are produced they may be transported for 

 considerable distances by currents. Thus the pelagic 

 larvae of the crustacean Squilla may be found in 

 St George's Channel but we have no record of the 

 occurrence of the adult there. It probably reproduces 

 only in the Biscay-Mediterranean waters and the 

 larvae are carried to the north in the Gulf Stream 

 drift. So also with the copepods which are occasion- 

 ally found in the Faroe-Shetland Channel: their 

 productive area is in tropical or sub-tropical seas and 

 the larvae are simply drifted into northern waters, 

 growing but not reproducing there. If such pelagic 

 larvae were to live long enough there would probably 

 be no limit to their area of nominal distribution. 

 Powerfully swimming nektic animals or migratory 

 birds are also able to migrate far beyond their area 

 of optimal conditions ; thus sturgeon, breeding only in 

 the great rivers flowing into the Baltic may never- 

 theless be found over practically the whole north 



