THE LAW OF THE MINIMUM IN THE SEA 195 



which abound there. Here and there in the literature of 

 the voyages of explorers we meet with the expression of 

 astonishment at the wealth of life contained in the colder seas. 

 Darwin was particularly impressed with the abundance of life 

 in the sea off the coast of Tierra del Fuego — all the more 

 because the land was almost desolate. In those seas the great 

 kelp (Macrocystis pyriferd) grows up from a depth of forty-five 

 fathoms at the least, and the thallus of this alga is probably 

 much longer than this. On it is an incredible abundance of 

 invertebrate life, the food of fishes and marine birds. " I can 

 only," says Darwin, " compare these great aquatic forests of 

 the southern hemisphere with the terrestrial ones of the inter- 

 tropical regions. Yet if in any country a forest was destroyed, 

 I do not believe that so many animals would perish as would 

 here from the destruction of the kelp." x 



Just the same contrast was observed when the Kiel plankto- 

 logists made quantitative observations of the abundance of the 

 plankton in the various seas, and these results are all the more 

 significant since they can be expressed numerically. Plankton 

 catches made with the Hensen apparatus showed that the 

 largest catches of diatom life were made in the sea off the 

 north-west shores of Greenland. Nowhere was there such 

 abundance of plankton in the sea as in this area. For several 

 years systematic observations were made in the Baltic in Kiel 

 Bay, and here again large catches of diatoms were made, 

 though not so large as in the Arctic Sea. For comparison 

 with these, catches made in the Mediterranean might be quoted. 

 In these seas the plankton was everywhere less abundant 

 than in the colder Baltic and North Seas ; so also with the 

 plankton of the warm Sargasso Sea of the Atlantic Ocean. 



What is the cause of this contrast ? As we pass from the 

 equator to the poles both the temperature and the intensity of 

 light decrease on the land and in the sea ; and we should expect 

 to find a corresponding decrease in the abundance of life if 

 these factors were the only ones to be taken into account. 

 It is true that the extremes are not so far apart in the sea as 

 on the land. The range of temperature on the land is from 



— 66° to about 66° C, but in the sea the range is only from 



— 2 , 8° to 31 C. The temperature of the sea is therefore more 

 uniform than is that of the land : the change in the intensity 



1 Voyage of the "Beagle." 



