TIME AND ANIMAL COMMUNITIES 97 



spring from the snow-hole in which he has been hibernating, 

 he has at first to eat seaweed, and a little later on may be seen 

 actually grazing. In the middle of June the salmon start 

 to come up the rivers from the sea, and from this time onward 

 salmon forms the bear's staple diet. In August he also eats 

 large quantities of wild peas which are then abundant, and in 

 September, berries. Finally, in the late autumn, he goes and 

 digs up ground-marmots (susliks) in the hills. Having 

 accumulated enough fat to last him through the winter he 

 retires into hibernation again and lives on it in a comatose 

 condition until the following spring. 



Many foods are available only at certain times of the year, 

 and this results in the formation of certain temporary niches 

 which may be at the same time world-wide in their distribution. 

 For instance, in Britain the raven (Cormis corax) feeds in 

 spring upon the placentae, or afterbirths, of sheep, ^*^ and on 

 the Antarctic ice-pack MacCormick's Skua (Megalestris 

 MacCormicki) eats the afterbirths of the Weddell seals. ^-^ 



22. On the other hand, the difference of physical and 

 chemical conditions may be so great that the same niche is 

 filled by different species, often of the same genus, at different 

 times of the year. There are a number of copepods of the 

 genus Cyclops which live commonly in ponds, feeding upon 

 diatoms and other alg^e. In winter we find one species, 

 Cyclops strenuus, which disappears in summer and is replaced 

 by two other species, C. ftiscus and C. albidus, the latter, 

 however, disappearing in the winter. There are also, however, 

 some species of Cyclops which occur all the year round, e.g. 

 C. serratulus and viridis. Instances of this kind could be 

 multiplied indefinitely, not only from Crustacea, but also from 

 most other groups of animals, and many will occur to any 

 one who is interested in birds. 



23 . We started with the conception of an animal community 

 organised into a complicated series of food-chains of animals, 

 all dependent in the long run upon plants, and we showed 

 that each habitat has its characteristic set of species, but at 

 the same time retains the same ground plan of social organisa- 

 tion. When the factor of time is introduced it is immediately 



H 



