CHAPTER III 



THE FACTORS OF DISTRIBUTION 



IF we were to compare lists of the species of 

 plants and animals known to occur in the English 

 Channel, the Irish Sea and the Firth of Clyde- 

 adjacent sea areas which have been fairly well 

 investigated, we should find that the majority of the 

 kinds of organisms were common to all three regions, 

 but also that each of the latter possessed a few species 

 which did not occur in one or more of the others. 

 We should also find that certain species were most 

 abundant in one of the areas and decreased in density 

 as we passed north or south. If again we were to 

 compare the fauna and flora of the Indian Ocean off 

 the coasts of Ceylon with those of the North Atlantic 

 between the Faroe Islands and the coasts of Scotland 

 we should find quite the opposite kind of contrast : 

 the great majority of the species would be different 

 in each case, yet there would be a few which were 

 common to the two areas. If, further, we had ex- 

 haustive lists of the animals and plants found in the 



