STRUCTURE AND DEVELOPMENT 41 



forces the results already established. After what has 

 been said, it will not be difficult to perceive the meaning 

 of the resemblances among mice of the house and field, 

 and of rats and rabbits and squuTels. All of them 

 possess heavy curved gnav/ing teeth, or incisors, and 

 lack the flesh-tearing or canine teeth. They agree 

 in many other respects which distinguish them as a 

 separate natural order of the mammals called the 

 rodentia. Again we find a highly aberrant form in the 

 flying squirrel, which leads toward an order with another 

 plan of body. This animal is a true rodent, which 

 lengthens its leap from branch to branch by means of a 

 fold of skin stretching between its fore and its hind 

 limbs. It is an animated aeroplane, and it shows in 

 part how bats have originated. The wing of a bat is an 

 elastic membrane stretching not only between the two 

 legs of one side, but also between the greatly lengthened 

 *' fingers '^ of the fore limb. But the bones of arm, wrist, 

 and fingers are almost precisely the same in number 

 and relation as in walking forms. The fact that this 

 peculiar wing adheres to a plan belonging to the anterior 

 legs of walking or climbing types has no reasonable 

 explanation save that of evolution. 



The well-known group of hoofed animals, including 

 horses and cattle, is also valuable for our present pur- 

 poses, as well as in a later connection when the evidence 

 of fossils is described. The elephant possesses five toes 

 armed with well-developed nails or hoofs. A tapir has 

 four or three toes, and it would seem that its ancestor 

 had had five toes, of which one or two had been lost. 

 A rhinoceros possesses three toes, and its foot is con- 

 structed internally like the elephant's with the outer 



