38 Instinct and Memory exhibited by the [ January, 
others, but I have not observed it in my reading. Have we not 
in the modified structure of the flying squirrel, and in the tremor 
of its fore legs while sailing, the true key to that further modifi- 
cation in the bat which gives it the power of flight? 
The common squirrels when they jump from any considerable 
height to the ground, have the habit of extending the legs in the 
manner of the flying squirrel, and at the same time of broadening 
the body very much horizontally; this is of manifest account in 
reducing the energy of impact due to the fall, and suggests pos- 
sibly both the method and the occasion for the modification now 
possessed by the flying squirrels. The traction brought to bear 
upon the integument between the limbs in the effort to spread 
the legs, must stretch it, and may be supposed to have begun a 
modification which was perpetuated and intensified by natural 
selection until the modification in the flying squirrel was reached. 
The habit of spreading the legs may have had its origin partly in 
the mere effort to balance the body and maintain the desired atti- 
tude for alighting, and partly in the knowledge obtained experi- 
mentally in repeated acts of jumping. 
It does not seem improbable that the development of wings in 
the bat may have been initiated in the same manner and have 
passed along essentially the same road, that is, the earlier ances- 
tors of the bats may have had a dermal modification nearly iden- 
tical with that of the flying squirrels, and which may have been 
used in much the same way for similar purposes. The next step, 
probably, in the development of the bat’s wings, was the forming 
of the habit of vibrating the fore limbs together in a vertical 
plane, and the embryonic phase of that movement, it seems to 
me, may be represented in the tremor of the limbs mentioned as 
occurring in the flying squirrel under consideration. 
In the effort to maintain the proper attitude of the body, we 
may have had the initiating factor; for if they were originally 
provided with parachute-like appendages, and used them as the 
flying squirrels do, it is probable that a similar vibratory move- 
ment would have been a necessity in order to keep the body in 
the attitude which would present the greatest surface to the air in 
falling. With the vibratory habit fixed, increased skill in execut- 
ing it would of necessity prolong the leaps, and this is another 
step towards flying; and increased use.and greater advantage 
Se ee: 
De eee Nee Pe ey ees Ty SS 
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REAA eg S E A 
