240 THE CONDITIONS OF LIFE IN THE SEA [PART III 



from time to time. But further investigation will shew that 

 certain physical factors — the temperature, degree of concentration 

 of the salts of sea water, and the intensity of sunlight — are also 

 important factors. Hydrography — the study of the physics of 

 the waters of the sea — is now an important department of general 

 marine biology ; and the results of this branch of marine research 

 must be considered in all discussions of the question of the 

 variations in the nature and abundance of life in the sea. It is 

 astonishing that it should have been very difficult to induce marine 

 zoologists to take the view that the distribution of species of 

 animals and plants, is in the long run the effect of physical factors; 

 and that hydrographical research is necessary if we are to learn 

 what are the causes of the variations in abundance, and the 

 migrations of marine organisms. One is so accustomed to think 

 of corresponding biological phenomena on the land as dependent 

 on climatic conditions that it is extraordinary that it should be 

 difficult to think similarly with regard to the sea. Mammals, 

 reptiles, some insects, worms and so on live on the surface of the 

 land, at the bottom of the atmosphere, just as benthic animals 

 live at the bottom of the sea (the hydrosphere) ; and birds and 

 many insects may be compared with the pelagic animals which 

 live in the free waters of the sea. It is an every-day experience that 

 the growth of crops and other terrestrial plants, and the breeding 

 seasons, growth, and migrations of terrestrial animals such as 

 birds, are ultimately dependent on the physical condition of the 

 atmosphere (climate), and yet one fails to realise strongly that 

 corresponding factors in the sea must have similar effects. 

 Temperature, rainfall, and the hygrometrical condition of the 

 atmosphere all affect the life-processes of the animals and plants 

 living on the surface of the land, and apart from actual evidence, 

 one would conclude that temperature and salinity would also 

 affect the habits and metabolism of marine organisms. But in 

 the past marine biology has been studied mainly from the point of 

 view of speciography and morphology, while hydrography has been 

 studied quite apart from biology. It is only within the last two 

 decades that our knowledge of the connection between the 

 life-histories of marine animals and the hydrographical condition 

 of the sea has been acquired, and indeed it is still very meagre. 



