556 Causes and Course of Organic Evolution 



ologists, the body of most birds is constructed on a more per- 

 fect plan than even that of man. But a speciaHzed body plan 

 is one thing, a means for stimulating brain activity and brain 

 increase is a very different one. This will be shown to apply 

 strikingly in the present instance. 



For the great majority of birds environal contact is effected 

 mainly by the primary sense organs, specially those of sight, 

 sound, and in a few doubtful cases of smell. But the bill also 

 in relation to the actively moving head is often a most efficient 

 conductor of tactile and possibly also of chemotactic stimuli. 

 The accessory appendages here however, in the shape of fore- 

 limbs, have become wholly modified for the one function of 

 air motion. But this one mode of stimulation of the brain, 

 and reflex action from it, is of very limited range in variety. 

 In the most mentally alert however, like the parrot, the crow, 

 the jackdaw, and others, one foot is often used as a manual 

 organ to grasp, to tear, to weave, to roll, to climb, and in con- 

 junction with the bill to carry on varied acts, specally those 

 connected with nest-building. This is in marked contrast to 

 the other group of Sauropsida — the Reptilia — in which the 

 limbs are purely organs of locomotion or are atrophied. The 

 above-named birds, then, rank as the most intelligent and re- 

 sourceful of their class. 



But in birds the exceptional powers of flight have acted so 

 as markedly to enlarge and complicate the cerebellum over 

 what we find in reptiles. This has doubtless been aided, on 

 the principle of action and reaction, by the increasingly quick 

 alert movements, when they alight on the ground, and which 

 shelter them from the attack of other animals. But though 

 birds have evolved along an entirely distinct line from the 

 mammals, and in some respects along a higher line, their brain 

 has remained decidedly smaller relatively; it is also simpler, 

 smooth, and less subdivided into cerebral areas. This result 

 we would interpret to be largely if not wholly due to the modi- 

 fication of the forelimbs into wings, and the absence of any good 

 accessory appendage, except in some cases one foot, for aiding 

 in the rapid conveyance of varied stimuli to the brain. 



