DISPERSAL 155 



adufictis, and Chydoriis sphcericus). The island was frequented 

 by enormous numbers of birds, and since these species of 

 water-fleas are all littoral ones and furthermore form winter 

 eggs which easily become entangled with birds* feathers, he 

 concluded that the birds had been the transporting agent. 

 This is usually the sort of evidence which one gets about 

 accidental animal dispersal ; the final proof is seldom forth- 

 coming, owing to the difficulty of catching the animals in the 

 act, especially when the dispersal may be a rather rare event 

 in any case. 



9. One is struck by the way in which the true and often 

 quite important facts about animal dispersal, and especially 

 accidental dispersal, are usually met with when one is work- 

 ing on something else in the field. In fact, it is almost impos- 

 sible to spend any time profitably in a deliberate study of 

 animal dispersal as such, except in certain well-defined direc- 

 tions, as in seasonal migrations of birds and fish, or parasites, 

 or species whose larvae regularly hang on to a definite host. 

 Thus the writer has found a frog carrying a fresh-water bivalve 

 (Sphcerium) attached to its hind leg, while the real work on hand 

 was the collecting of neotenous newts. And while studying 

 the habits of rabbits in Herefordshire, he has suddenly noticed 

 that hordes of young toads were migrating across the country 

 in the direction of the local lake, and was surprised to find 

 that they were making for a piece of water which they were 

 unable to see, since it was hidden by a high wall. It seemed 

 clear that they must be responding to the humidity of the air ; 

 but even that seems a little hard to believe, when one considers 

 that they were more than a hundred yards from the water. 

 It is on such facts as these that our knowledge of animal dis- 

 persal will always have to be built up, facts which are usually 

 encountered by accident, in a casual way, and often in circum- 

 stances which make it impossible to follow up the problem 

 any further. It is therefore especially desirable to publish 

 such notes on dispersal, however fragmentary they may be, 

 since there is not usually much chance of getting better ones 

 for some years. And it may be noted here that there is a 

 certain reluctance among zoologists to publish incomplete 



LU ' L ! B R A R Y 



