FLIGHT 



83 



These four types of wing are analogous, for all of them 

 are used in actively striking the air. But the wings of 

 insects, which originate as hollow outgrowths from the 

 dorso-lateral regions of the two posterior segments of the 

 thorax (mesothorax and metathorax), are not in any way 

 comparable, as regards development or structure, to the 

 wings of Pterodactyl, Bird, and Bat which are transformed 

 fore-limbs (and rather more in Bats). This is expressed 

 by saying that the wings of insects are analogous but not 

 homologous with the other wings. On the other hand, if 



MC1 



Fig. 21. — Skeleton of bat, creeping, sc, scapula or shoulder-blade ; 

 CL., clavicle or collar bone ; h., humerus or upper arm ; R., radius of fore- 

 arm ; MCI, metacarpal of the thumb (th.) ; mc.2, metacarpal or palm 

 bone of second digit ; f., femur, or thigh bone ; AC, special heel bone. 



we keep to the main part of a bat's wing, we may say that 

 the wings of Pterodactyl, bird, and bat are both homologous 

 and analogous with one another, being transformed fore- 

 limbs and organs of true flight. If we compare any of 

 them with our own fore-limb, we see a general similarity 

 in structure and mode of development, but there is no 

 analogy. 



These distinctions, first made clear by Sir Richard 

 Owen, are not mere pedantries, but enter deeply into the 

 interpretation of organic structure. Let us take Sir Richard 

 Owen's examples : — 



