(Cee) 
group, and may have then separately ‘‘converted ’’ each new- 
comer as it arrived in the district. The tendency of a group 
was certainly, as Mr. Blandford maintained, to extend its 
limits indefinitely—a tendency which had operated with great 
success in certain cases. But the spread of species always 
encountered opposing forces which in many cases acted as 
effective barriers. 
With regard to Brunner’s ‘ Hypertely,” he maintained 
that one knew far too little of the details of the struggle for 
existence to justify the conclusion that it was incompetent to 
produce such effects. What little was known confirmed the 
belief that very minute differences might serve to turn the 
scale. The differences between extremely perfect resem- 
blances and those which were less perfect or only very rough, 
were probably to be explained by the relative age of the 
association in the former, or the more complete and rapid 
operation of natural selection on account of a special reliance 
on this among other modes of defence possessed by the 
species. One was compelled to believe that every perfect 
resemblance began as an imperfect resemblance, and then 
passed through stages in which the likeness was gradually 
increased ; and it was only to be expected that examples of 
all such stages should exist at the present day among the 
numberless forms which exhibited mimicry and common 
warning colours. 
Gradual changes in the geographical distribution of the 
constituent species along the borders of groups would tend 
from time to time to bring certain of them within the 
influence of other groups, and so begin a change in another 
direction. Furthermore, there was no reason for concluding 
that the detached members of Miillerian groups must become 
extinct, as in the case of the Batesian or true mimic. In the 
presence of other dominant members of a group, any ten- 
dency towards resemblance might well be of selection value : 
in their absence it was by no means necessary to assume that 
a species, which, ex hypothesi, was specially protected in some 
way, must become extinct, although any further advance 
towards. the likeness would be checked, and the ground gained 
in the past lost after a longer or shorter interval. To enter 
