136 MISS MARION I. NEWBIGIN. 
happens when some change of current uncovers a ridge of 
rock hitherto concealed by the sand. In such an area it is 
easy to observe the gradual process of colonisation which 
occurs. First come Alge of various kinds, the coarser 
varieties being usually the first to gain a foothold. Then 
acorn-shells and herbivorous molluscs make their appearance, 
and as these thrive and multiply, they are followed. by 
carnivorous whelks and star fishes. As the weeds grow, 
crabs and other crustacea appear; and the new settlement 
thrives apace until it contains most of the animals inhabit- 
ing the parent area. How the animals reach the new area 
is a question to which we shall return later, meantime we 
are concerned with the causes which determine the gradual 
process of colonisation, and the reason why it only occurs 
where there is a solid substratum of some kind. We find 
that in both cases the answer is very obvious, it is essentially 
a question of food. Before shore animals can live and 
multiply there must be an abundant supply of weed; for if 
the flesh of all land animals is grass, the flesh of all marine 
animals is sea-weed. Consider this for a moment in detail. 
Periwinkles and limpets live on alge ; whelks, dog-whelks, 
and their allies live on periwinkles, limpets, and similar 
forms; the fish eat the whelks and dog-whelks; the seals 
and porpoises eat the fish. So with any other chain of 
organisms, the first link upon which all depend is always the 
humble weed. Now the marine alge are not like land 
plants, they have no true roots, and are able to obtain all 
necessary food from the water; but save under exceptional 
circumstances they cannot flourish without a firm substratum 
to which they may attach themselves, and such a substratum 
is furnished under ordinary conditions only by rocks. 
Those who are botanists may, however, quite naturally 
object that not all alge are large or attached, that the above 
description leaves out of account the diatoms and numerous 
other microscopic algze which form the basal article of diet 
for the animals of the open sea, and which must also be an 
important source of food for littoral animals. This is quite 
true, and we have to include in the food supply of shore 
animals, not only these microscopic alge, but also the 
particles of various kinds swept into the sea by our rivers 
