2 LIFE IN THE SEA [CH. 



be difficult to devise a grouping which will be at once 

 convenient and philosophical. 



Let us suppose that it is possible to start from 

 some place on the west coast of Ireland and walk out 

 along the sea-bottom, underneath the North Atlantic 

 Ocean, until we reached the opposite shore of North 

 America. As we did so we should witness a re- 

 markable series of changes in the physical condition 

 of the sea-bottom, and of the water over our heads ; 

 and we should also witness a similar change in the 

 nature of the life of the sea ; such a change as would 

 enable us to associate the variations in the physical 

 conditions with certain variations in the abundance 

 and nature of the animal and plant life of the ocean. 

 We will suppose that we were to start out from some 

 stretch of beach covered by stones and gravel: on 

 the higher parts of such a beach we should find a 

 particular kind of life large sea- weeds, such as 

 the bladderwrack, clinging to the stones; mussels, 

 barnacles, periwinkles, dog- whelks and small Crustacea 

 such as the sandhoppers. The animals and plants of 

 which these are the most familiar examples inhabit 

 the sea beach between the upper and lower tide 

 marks and we may call them the Littoral Benthos. 

 Now descending the beach to near the limit of low 

 water of high spring tides we should find a more 

 varied and luxuriant fauna and flora, which would 

 consist of the large-fronded sea-weed Laminaria, 



