692 



THE POPULAR SCIENCE MONTHLY. 



Now, what is the meaning of these relations between organs and 

 between animals ? For that they have a meaning must be clear to all 

 and it is fair to presume that it is one that can be discovered by in- 

 vestigation. 



The fact that two or many different animals are constructed on the 

 same plan seems to indicate some kind of connection between the 

 animals themselves, and it is the work of the zoologist to find what it 

 is that thus connects them. 



Fig. 5. 



Fore-foot, or "Wing," or Embryo Yellow 

 Warbler.— (From Morse.) The hand in a 

 Reptile and Embryo Bird compared. U, ulna ; 

 B, radius ; «, ulnare, or cuneiform bone ; r, 

 radiale, or scaphoid bone ; c, centrale. 1, 2, 

 8, 4, 5, first, second, third, fourth, and fifth 

 carpals, m 1 , nfi, m 3 , m*, m 5 , corresponding 

 metacarpals. 



Fig. 6. 



Fore -foot, or "Paddle," or 

 Snapping - Turtle. — (From 

 Gegenbauer.) 



Two theories have been proposed, each of which seems to meet 

 most of the points to be explained, but each seems to fail in some re- 

 spects. One of these is, that the connection between different groups 

 of animals is to be found only in the mind of their Creator; the other 

 is, that there is a direct genetic connection or relationship between 

 them. 



Each of these theories is conceivable and worthy of considera- 

 tion, for we can find examples of the building up of systems some- 

 what resembling the animal kingdom in each of these ways. The 

 various kinds of steam-engines, for instance, are adapted each to its 

 special work, with an accuracy rivaling that of Nature, yet all of 

 them can be shown to be constructed on substantially the same plan. 



If we trace the history of any form, such as the steep-grade loco- 

 motive, we find, as we go backward, that it loses, one by one, all of 

 its special adaptations, until at last it is only a common locomotive 

 at up-hill work. Tracing the history of the locomotive in the same 

 way, we find that its special adaptations disappear, until it is nothing 



