158 ANIMAL ECOLOGY 



Land (which is about the size of Wales). Since the surface 

 is entirely covered with ice and snow, or else consists of very 

 barren rocks with a high-arctic flora, there was not the remotest 

 chance of either the aphids or the hover-flies surviving. As a 

 matter of fact, the majority of them were wiped out by a 

 blizzard which occurred three days later. 



12. Abortive colonisation is happening everywhere in 

 nature, on a smaller scale, and the two examples quoted above 

 are given in order to drive home the fact that dispersal by 

 itself may have absolutely no effect upon the distribution of 

 a species, unless it is accompanied by eifective establishment 

 at the other end. The factors controlling the survival of the 

 species have been dealt with in a general way earlier in this 

 book, and it is therefore unnecessary to say more about 

 them here. We may note, however, that one of the chief 

 difficulties facing an animal upon its arrival is that of finding 

 a mate, with which to co-operate in perpetuating its race. The 

 chances of one individual copepod reaching a new piece of 

 water by accidental dispersal may be small, and that of two 

 individuals of opposite sexes doing so smaller still. But the 

 chance of these two meeting and mating and bringing up 

 young would often seem to be extremely remote indeed. We 

 find, therefore, that animals which have to reach new places 

 by definite migration or other special means of dispersal are 

 either parthenogenetic during part of their life-cycle, or else 

 migrate in large parties, like locusts. In the case of locusts 

 there are well known to be special tropisms which cause the 

 animals to stay together in swarms, and to follow the move- 

 ments and flight of their neighbours, so that there is a good 

 chance of a swarm of both sexes arriving at the other end.^^ 

 In the case of most water-fleas the egg which is transported 

 by accident hatches into a female, which is able by par- 

 thenogenesis to produce a huge population, amongst which 

 males occur at a later stage. Tliis is also a common method of 

 colonisation used by parasites, whose Hfe-history is often so 

 risky that the chance of two animals of opposite sexes arriving 

 together in one host is negligible. 



13. With regard to the methods employed by animals to 



