DISPERSAL T.Y MAN. 1 83 



with accounts of such doings, but we are not much con- 

 cerned with this kind of dispersal. Numerous unsuc- 

 cessful attempts at colonization have been recorded, and 

 these are of significance, I think, as helping us to under- 

 stand how very small must be the chance of the ultimate 

 establishment of a new colony as the result of trans- 

 portal — often, no doubt, to very unsuitable spots — of a 

 solitary specimen or a (e\v individuals by accidental 

 means under nature. Man often carries considerable 

 numbers, turning them out in localities apparently well- 

 suited to the requirements of the particular species he 

 wishes to acclimatize, sometimes even watching over 

 them with care and shooting the birds which come to 

 pick them up, yet it often happens that the creatures 

 gradually decrease in numbers and finally die out. It 

 must be remembered, however, that when thus carried 

 by man they are generally put down in districts already 

 well-stocked, and the creatures in such cases are ob- 

 viously less likely to survive than those w^hich happen 

 to be transported by natural means to poorly stocked 

 regions or to newly formed and unoccupied islands. 

 The few accounts of attempts at colonization which I 

 have noted down may possibly be w^orth giving. Un- 

 successful attempts, it should not be forgotten, are 

 probably less likely to be placed on record than suc- 

 cessful ones. 



The zebra mussel [JDrcissaia polyvwrpJia) is said to 

 have been planted by Mr. Stuchbury of Bristol in some 

 of the waters near that place,^ and similarly, in America, 



' H. E. Strickland, "Mag. Nat. Hist.," (n. s.), ii., (1838), p. 363. 



