394 SECTIONAL TRANSACTIONS.—D. 
malian structures are accommodations to this habit. Whether these move- 
ments be reversible or irreversible, their objectives suggest some of the 
factors which have determined the great seasonal and reversible movements 
properly called migrations. In their limited way the seasonal migrations 
may be likened to the migrations of birds, for some are only local affairs, 
while others cover a wide range ; but birds pass without record, while many 
mammals create regular migration tracks or roads which they follow year 
after year. ‘The migrations of bats, which ought to form the closest 
parallel with bird movements, do not appear to be essentially different from 
those of other mammals. In a different category from migration proper 
must be placed those occasional eruptions of such as the lemming in 
Scandinavia, the brown rat of Asia, or the grey squirrel and marten of 
North America, which are one-way pressure movements due to abnormal 
increase of numbers. 
Dr. N. A. MackintosH.—The migration of whales (10.25). 
It is generally assumed that the whalebone whales of the southern hemi- 
sphere spend the summer on the feeding grounds of the Antarctic, and 
migrate in winter into warmer waters where breeding takes place. Evidence 
of these migrations is partly derived from direct observations on the move- 
ments of the Humpback off the African coasts, and partly from indirect 
sources such as fluctuations in the abundance of Blue and Fin whales in 
cold and temperate waters, variations in the fatness of whales, and the 
existence of certain scars contracted in warmer regions but distinguishable 
on whales found in the Antarctic. Direct evidence is also forthcoming 
from the Discovery Committee’s marking experiments. 
It is difficult to distinguish between the long seasonal migrations and the 
various movements connected with changing ice conditions and _ local 
wanderings in search of food. Whales travel long distances, for instance, 
on the retreat of the pack ice in summer. 
The means by which whales orientate themselves during migration is not 
known. They are evidently sensitive to small changes of temperature, but 
this seems inadequate to guide them on a long migration. 
Dr. A. LaNDsBorouGH THOMSON, C.B.—The migration of birds (10.40). 
Annual migrations are common among birds, from wanderings in which 
the breeding place is the only fixed point to extensive journeys between 
widely separated seasonal habitats : there are also some less regular move- 
ments. In its more highly developed form, bird-migration shows a com- 
bination of complexity and regularity : inter alia, some remarkable examples 
of migration in purely tropical species are now known. Much evidence 
is available to show that migration is not performed merely under the 
compulsion of external forces, but commonly anticipates—and often 
apparently exceeds—the requirements arising from seasonal change in the 
environment. 
The inevitable assumption that migration is the expression of an instinct 
raises questions of causation. Migration must serve useful ends, and recent 
investigations have helped towards a closer definition of these. Its origin 
and development necessarily remain matters of speculation. The nature 
of the recurring stimuli which annually evoke the behaviour has, however, 
been the subject of interesting experimental work in the last few years. 
The final problem is that of orientation, including the manner in which 
the path to be followed is determined. 
