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tlie pressure of circumstances, to wit, sudden change of tem- 
perature and failure of subsistence, compel them to sacrifice their 
offspring for their own own safety. 
A remarkable circumstance in connection mth birds’ migra- 
tions is the regularity of their comings and goings. These are 
well estimated on the shores and islands of the Mediterranean 
at the spring and autumn equinoxes, when the vast hosts of 
birds of passage are on the move to and from their winter 
retreats in Northern Africa. Then one can almost calculate 
on the certainty of the arrival of many species which make 
either a lengthy sojourn, or only wait for a few hours to rest 
themselves. Some never halt, and are seen steering their 
courses at high elevations, evidently aware that the interme- 
diate lands, beyond a mere resting-place, are not those they 
are seeking. Some birds recognise persons and objects after 
a length of time ; indeed, the same individuals have been known 
to return to their nests year after year, as in the case of the 
swifts marked by Dr. Jenner.* Such instances are all the 
more remarkable that they occur in a bird in no way super- 
excellent in other mental acquirements, and of its tribe goes 
furthest into the interior of Africa during the winter months. 
The swifts, however, besides their almost unexampled powers of 
wing,f have a greater range of vision ; so that, supposing the 
English home is readily discernible by certain physical cha- 
racters, such as a particular mountain range, and the distance 
between it and the winter retreat is travelled over in a few 
hours, without a stoppage on the way, all that the bird has 
to treasure in memory is the general feature of the district 
it has left. This, however, has to be retained for nearly eight 
months, through daily-changing fortunes ; and even when the 
landmark is re-discovered, still another mental effort is re- 
quired before it finds out the nest of the previous year. No 
doubt winds and temperature, in connection with coast lines, 
&c., assist migrating birds in finding their way to and from 
their summer and winter retreats, and very likely certain 
mental powers are acquired and improved thereby; but the 
crediting of such an effort as the above to instinct alone is 
apparently incomprehensible. The desire to change residence, 
and the direction of the route, are indicated by seemingly 
natural causes, and there may be an instinctive disposition in 
every migratory bird to pursue a definite extent of journey ; 
but in the face of wind and weather on the one hand, and the 
* See Yarrelfs “ British Birds,” vol. ii. p. 234. 
t Spallanzani estimated the rapidity of the swift at about 276 miles an 
hour, or three times greater than the swallow. Thus, according to his cal- 
culation, the former would traverse the distance between Great Britain and 
its winter haunts in 20° lat. in less than seven hours. 
