84 The Outline of Science 



the square inch at a depth of 2,500 fathoms; of profound calm, 

 unbroken silence, immense monotony. And as there are no plants 

 in the great abysses, the animals must live on one another, and, 

 in the long run, on the rain of moribund animalcules which sink 

 from the surface through the miles of water. It seems a very 

 unpromising haunt of life, but it is abundantly tenanted, and it 

 gives us a glimpse of the insurgent nature of the living creature 

 that the difficulties of the Deep Sea should have been so effec- 

 tively conquered. It is probable that the colonising of the great 

 abysses took place in relatively recent times, for the fauna does 

 not include many very antique types. It is practically certain 

 that the colonisation was due to littoral animals which followed 

 the food -debris, millennium after millennium, further and further 

 down the long slope from the shore. 



The Freshwaters 



4. A fourth haunt of life is that of the fresh-waters, includ- 

 ing river and lake, pond and pool, swamp and marsh. It may 

 have been colonised by gradual migration up estuaries and rivers, 

 or by more direct passage from the seashore into the brackish 

 swamp. Or it may have been in some cases that landlocked 

 corners of ancient seas became gradually turned into freshwater 

 basins. The animal population of the freshwaters is very repre- 

 sentative, and is diversely adapted to meet the characteristic con- 

 tingencies the risk of being dried up, the risk of being frozen 

 hard in winter, and the risk of being left high and dry after floods 

 or of being swept down to the sea. 



Conquest of the Dry Land 



">. The terrestrial haunt has been invaded age after age by 

 contingents from the sea or from the freshwaters. We must 

 recognise the worm invasion, which led eventually to the making 

 of the fertile soil, the invasion due to air-breathing Arthropods, 



