182 BULLETIN OF WISCONSIN NATURAL HISTORY SOCIETY. VOL. 2. NO. 4. 
"I believe it was only some twenty or thirty years ago that 
anything- like a practical solution of the difficulty was arrived at. 
The birds congregating about the south coast (i. e. of England) 
are seized with a sudden impulse, or mania, to fly upwards. This 
is caused by some atmospheric change, coinciding with a warm 
south wind moving in a high stratum, into which the birds soar 
with an involuntary motion of the wings. This motion (involun- 
tary like that of the heart) is continued for many hours, and the 
birds fly blindly along until the paroxysm passes off, when they 
at once begin to descend, making many a fatal drop into the 
sea" (13). 
The results of Palmen's work, however, threw light upon the 
causal phenomena of migration, and we have the appearance at 
about this time of two excellent theories explaining the origin 
and causes of migration. Although various influences connect- 
ed with this phase of the work had been correctly hinted at by 
Darwin, Baird (14), Newton (15), Hutton, and others, it was not 
until 1874 when Dr. Alfred R. Wallace advanced an explanation 
that we have had a really excellent theory in complete and con- 
cise form. This was about the time when Palmen's work appeared r 
whether Wallace was influenced by the work of the latter or not 
is difficult to say. If not he certainly anticipated something of 
the nature of his results. Another theory appeared soon after 
from the pen of Dr. August Weismann, who tells us that he fully 
accepts the results derived by Palmen. 
Wallace writes, 'Tt appears to be probable, that here, as in so 
many other cases, survival of the fittest will be found to have had 
a powerful influence. Let us suppose that in any species of mi- 
gratory birds breeding can, as a rule, be only safely accomplished 
in a given area ; and further, that during a great part of the rest 
of the year sufficient food cannot be obtained in that area. It 
will follow that those birds which do not leave their breeding area 
at the proper season will suffer and ultimately become extinct ; 
which will also be the fate of those which do not leave the feed- 
ing area at the proper time. Now, if we suppose that the two 
areas were (for some remote ancestor of the existing species) 
coincident, but by geographical and climatic changes gradually 
diverged from each other, we can easily understand how the habit 
of incipient and partial migrations at the proper seasons would 
at last become hereditary and so fixed as to be what is called an 
13. Nature, vol. 10, p. 415. 
14. Am. Jour. Science, 18G6; p. 178 et seq. * 
15. Nature, vol. 10, p. 415. 
