EVOLUTION 1043 



some streams to a length of from twelve to fourteen feet, and with 

 stems as thick as one's wrist; the common spear-thistle, which is 

 from two to five feet high in Britain, formed in some districts vast 

 impenetrable thickets six to seven feet in height; brambles, briars, 

 and other weeds took possession of whole districts, and threatened 

 to choke out all other vegetation." This riotous exuberance is due 

 positively to the stimulating conditions as well as unembarrassed 

 opportunities of the new territory, and negatively to the absence of 

 the previous checks. The pruning shears of natural selection have 

 more or less ceased to act — ^for the time being. 



The popular interpretation of the rapid spread of introduced 

 plants, e.g. "weeds", in new countries, is that they come from 

 places where the struggle for existence is keener and where they 

 have therefore become particularly efficient. According to Willis 

 (1922, p. 7), "the real explanation, in all but a very few doubtful 

 cases, is that their spread is due to change of conditions. This 

 has usually been effected by man, who has often altered, or even 

 destroyed, the conditions under which many societies of plants 

 formerly flourished, thus giving a fair field to those new comers 

 that were suited to the new circumstances." 



(4) Many naturalists have come to the conclusion that the lessen- 

 ing of the stringency of natural selection after transport to a new 

 country has allowed an increase in the number of varieties, and the 

 survival of peculiar forms, which would have been speedily elimi- 

 nated in the original environment. Thus, if there are few enemies, 

 one might expect more numerous conspicuous variants, such as 

 albinos. Supporting this view, BuUer said of birds introduced from 

 Europe into New Zealand: "Albino sparrows are far more common 

 than they are in their native country; and even the skylark not 

 infrequently changes its sober dress for a yellowish- white one." 

 Here the careful work of G. M. Thomson (1922) is of great value. 

 In 1 89 1 he also found good reasons for concluding that conspicuous 

 colour-variations were on the increase among the progeny of the 

 rabbits introduced into New Zealand, and also among introduced 

 birds like sparrows, thrushes, blackbirds, skylarks, and starlings. 

 Thirty years later he definitely withdrew this conclusion, the fallacy 

 being that he was at first so busy looking for anomalous characters 

 that he met with many, and unconsciously exaggerated the ratio 

 of their occurrence. In 1917 he wrote: "I cannot find now any more 

 white, coloured, or white-feathered birds than were to be found in 

 1876. There are different varieties of birds in different parts, but no 

 variations seem to be now taking place more than occurred before." 

 Alluding more generally to the belief that the removal of checks 

 in the new country allows variation to proceed very rapidly, he 

 writes (1922, p. 513): "After nearly fifty years of fairly close 

 observation I have to state very definitely that such a belief has 



