The Tufted Duck in Scotland. 149 
several papers of a similar character to this one. Thus 
Capercaillie hens are the first to yield to pressure of popula- 
tion at a breeding centre, and by force of circumstances fly 
outwards and follow “lines of least resistance” or “avenues 
of escape.” They are followed by the males in course of 
time, when the impulse becomes too strong to resist; but 
they are not followed, because the males know where to look 
for them, but because they are influenced—or governed—by 
the same circumstances, illustrating the same Law in Nature, 
which in the earlier stage affected the female pioneers. So 
fully has this been recognised by Scandinavian authorities, 
when speaking of this periodical outburst of males from 
previously populated centres, that they say of the Capercaillie 
cocks, that they “jforflyga sig,’ or “fly they know not 
whither.” Itis not so-called “instinct” which enables the 
birds to find their partners, which have preceded them, but 
determining causes, outside of their will or “instinct ”—a 
Law of Dispersal which seems to me to result in a natural 
outcome of following what has been termed—“ the lines 
of least resistance.” It is not chance, for there is no such 
item. In course of time the areas along these “lines of 
least resistance” become fully populated, and in the earlier 
progressive movements, only the likelier haunts are occupied. 
Later, when pressure of population takes place, new lines of 
dispersal are opened out, say right and left of the principal 
avenues of escape, and so, onward, and outward, the waves of 
distribution pass. But knowledge of these localities outside, 
or to right and left of the more beaten tracks, can only be 
attained by individuals after some years—it may be less or 
more—of biannual migration in autumn and spring. In 
such a procession, many are left, but perchance few are 
chosen. The best colonists take up the new abodes, the 
weak, as a rule, go to the wall. 
Now, as regards the localities which appear to be most 
affected by the Tufted Duck, I believe that the following 
short description, in general terms, will suffice, at least to 
show the drift of my argument. So far as our facts teach 
us, the Tufted Duck affects lochs, lakes, or ponds of con- 
siderable size; also such as possess no great depths of water 
