DISTRIBUTION OP SHORE ANIMALS. 323 



Asterias rulens, is the Boreal form, and is replaced in the 

 Lusitanian region by the spiny A. glacialis. The latter 

 species is totally absent on the East Coast of England 

 and Scotland, where A. rubens is abundant, often extra- 

 ordinarily abundant. On the West Coast of Scotland 

 both species occur, but A. glacialis is not very common. 

 In the South-west of England both species are abundant, 

 but east of Plymouth A. glacialis disappears. In this 

 case the Lusitanian form seems to find it difficult to 

 oust the Boreal species even in the warm waters of the 

 West. The two forms show no very obvious differences 

 of diet. 



It is not possible to discuss in detail the distribution of 

 British marine animals, but we may say generally that a 

 form which occurs all round our coasts is probably a Boreal 

 form; one which is found only on the South and West 

 Coasts probably Lusitanian ; one confined to the North and 

 North-east probably Arctic. The study of distribution is 

 of great interest, and it is not necessary to travel over 

 wide areas to study it, for the differences between adjacent 

 areas are of as much interest as those between the ex- 

 treme North and extreme South, and illustrate the same 

 problems. 



The more attention you devote to problems of distribu- 

 tion, the more will you become impressed with the fact 

 which we have constantly endeavoured to emphasise, that 

 the shore is the region characterised essentially by its great 

 variability. If you study one area for a succession of years 

 you will notice how currents change, how deposits brought 

 down by rivers vary in character and distribution. Closer 

 observation is required to show that there are also gradual 

 variations in the salinity of the water, its clearness, tem- 

 perature, and so on, while the aid of the geologist must be 

 invoked to demonstrate the fact that the land is undergoing 

 slow oscillations of level, stable and changeless as it may 

 seem. Now these constantly changing conditions have a 

 most important effect upon the littoral animals, for they 

 induce relatively rapid variation. For example, the Firth 

 of Forth, from a multitude of causes, grows muddier year 

 by year. We know that muddy water is fatal to many 

 organisms, owing to its forming a deposit on their delicate 



