DISTINGUISHING CHARACTERS OF INSECTS. 17 



whether to call them hands or feet, while in the turtle 

 they are fins. The bird offers yet another variety, its 

 limbs consisting of two legs and two wings. Now a 

 glance at a skeleton will show how small a difference of 

 development in the proportions and direction of the bones 

 makes the difference between the foot and the hand 

 in man. It takes more study to find that the foot 

 of the horse is the foot of the man, saving that only 

 one enormous toe with a proportionate nail (the hoof) is 

 developed, the rudiments of but two others appearing on 

 dissection, while the rest are altogether atrophied. This 

 will appear less startling when the development is traced 

 next in the two-toed animals ("cloven-fqoted"), as the 

 cow, the three-toed rhinoceros, the pig with its four toes 

 two big and two little, the cat with its five-clawed 

 toes, one of which takes a direction separate from the 

 others like the human thumb. 



These examples must suffice : it would be out of place 

 in this work to trace the variety of development which, 

 from the same system of bones, produces the human hand, 

 the wing of a bird, and the hundred-fingered fin of the 

 skate ; or again, to trace the atrophy of parts by which 

 in reptiles the limb dwindles down to a mere indication 

 as in the slow worm to be altogether lost in the true 

 snakes. No line can be drawn between the highest and 

 the lowest of the vertebrata which shall separate them 

 from each other so clearly as it will be shown that they 

 are separated from all invertebrate animals. 



Passing from the framework of the body to the organs 

 by which the vital functions are performed, we find in all 

 the vertebrata a nervous system originating and centring 

 in the brain, whence, by ramifications from the spinal 

 chord, the whole body is supplied with nerves, the mys- 



c 



