154 ANIMAL ECOLOGY 



wood is an important agent of accidental dispersal. Beebe ^^^ 

 states that on one log collected out at sea during the voyage 

 of the Arcturus, he counted fifty-four species of marine 

 animals, including numbers of crabs, fish, and worms. In 

 modern times, ships act as the most gorgeous hollow floating 

 logs and have been used by animals on a large scale. 



8. Animals form the means of transport of a great many 

 other species. Apart from parasites, permanent or temporary, 

 there are a number of cases of accidental carriage which have 

 been recorded. Thus, Kaufmann^^^ noticed that the fresh- 

 water ostracod Cyclocypris Icevis^ which he was keeping in 

 an aquarium, had the habit of occasionally hanging on to 

 the legs of certain water-beetles, so that it is possible 

 that they may get about in this way from pond to pond. 

 Water-mites have the similar habit of hanging on to all manner 

 of insects (beetles, bugs, flies, etc.) in the larval state, and in this 

 way getting carried to new ponds. There are a number of 

 other examples which might be quoted, but to do so would 

 obscure the fact that in an enormous majority of cases we have 

 not the slightest idea in what way animals do commonly 

 get about. Take the case of Polyzoa living in ponds. It is 

 easy enough to conjecture that they may get about on birds' 

 feet or some such manner, but very difficult indeed to find an 

 actual case of their doing so. Only occasionally are we lucky 

 enough to catch them in the act, as when Garbini^^^ found the 

 statoblast or winter egg of Plumatella on the beak of a heron. 

 Often it is possible to obtain circumstantial evidence as to the 

 methods of dispersal, evidence which usually takes the form 

 of saying : " There are only two ways these animals could 

 have got to this island (or pond, or wood), and this one seems 

 the most likely." Thus Leege^^^ studied the fresh-water 

 Crustacea in a pool of water on an island off the Friesian coast, 

 an island which had only risen out of the sea during the pre- 

 vious ten years, and which must have obtained its fresh- water 

 fauna from elsewhere by accidental dispersal. In 1907 there 

 was no pool ; in 1908 it was formed, and in 1909 von Leege 

 found a large number of specimens of four species of common 

 water-fleas {Daphnia pulex^ Simocephalus exspinosus^ Pleuroxiis 



