NATURAL SELECTION, AND THE ANTIQUITY OF LIFE 223 



night and day by a vast army of dark-brown jelly-fish {Linergcs 

 merciitid), 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 centre-board well, and everywhere, to a 

 depth of fifty or sixty feet, we could see them drifting by in a 

 steady procession like motes in a sunbeam. We cruised through 

 them for more than five hundred 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 we dipped up ; nor is this abundance of life 

 restricted to tropical waters, for Haeckel tells us that he met 

 with such enormous masses of Limacina to the northwest of Scot- 

 land that each bucket of water contained thousands. The ten- 

 dency 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 windrow of fish half a mile wide and at least twenty 

 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 cope- 

 pods every day. 



In spite of this destruction and the ravages of armies of 

 medusae and siphonophores and pteropods, the fertility of the cope- 

 pods is so great that they are abundant in all parts of the ocean, 

 and they are met with in numbers that exceed our power of 

 comprehension. On one occasion the Challenger 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 cope- 

 pods are sometimes more than a mile thick. When we reflect 



