THE ORIGIN OF THE OLDEST FOSSILS. 365 



III the other great groups of marine animals we find some scavengers, 

 some which feed u[)on micro-organisms, and others which hunt and 

 destroy each other, but there is no group of marine animals which cor- 

 responds to the herbivora and rodents and the plant eating birds and 

 insects of the land. 



There is so much room in the vast spaces of the ocean, and so much 

 of it is hidden, that it is only when surface animals are gathered to- 

 gether that the abundance of marine life becomes visible and impres- 

 sive; but some faint conception of the boundless wealth of the ocean 

 may be gained by observing the quickness with which marine animals 

 become crowded together at the surface in favorable weather. On a 

 cruise of more than two weeks along the edge of the Gulf Stream I 

 was surrounded continually night and day by a vast army of dark- 

 brown jelly-fish {Linerges mercutia), whose dark color made them very 

 conspicuous in the clear water. We could see them at a distance from 

 the vessel, and at noon, when the sun was overhead, we could look down 

 to a great depth through the center-board well, and everywhere, to a 

 depth of 50 or 60 feet, we could see them drifting by m a steady pro- 

 cession like motes in a sunbeam. We cruised through them for more 

 than 500 miles and we tacked back and forth over a breadth of almost 

 a hundred miles, and found them everywhere in such abundance that 

 there were some in every bucketful of water which w^e dipped ui); nor 

 is this abundance of life restricted to tropical waters, for Ilaeckel tells 

 us that he met with such enormous masses of Limacina to the north- 

 west of Scotland that each bucket of water contained thousands. The 

 tendency to gather in crowds is not restricted to the smaller animals, 

 and many species of raptorial fishes are found in densely packed banks. 



The fishes in a school of mackerel are as numerous as the birds in a 

 flight of wild pigeons, and we are told of one school which was a wind- 

 row of fish half a mile wide and at least 20 miles long. But while 

 pigeons are plant eaters, the mackerel are rapacious hunters, pursuing 

 and devouring the herrings as well as other animals. 



Herring swarm like locusts, and a herring bank is almost a solid 

 wall. In 1879 three hundred thousand river herring were landed in a 

 single haul of the seine in Albemarle Sound; but the herring are also 

 carnivorous, each one consuming myriads of copepods every day. 



In spite of this destruction and the ravages of armies of medusje 

 and siphonophores and pteropods the fertility of the copepods is so 

 great that they are abundant in all parts of the ocean, and they are 

 met with in numbers which exceed our power of comprehension. On 

 one occasion the GhaUenger steamed for two days through a dense cloud 

 formed of a single species, and they are found in all latitudes, from the 

 Arctic regions to the Equator, in masses which discolor the water for 

 miles. We know, too, that they are not restricted to the surface, and 

 that the banks of copepods are sometimes more than a mile thick. 

 When we reflect that thousands would find ample room and food in a 



