358 



THE CHANGING GENERATIONS 



are nearly the same in all individuals. As the relationship grows less, 

 however, so does the degree of homology that exists. When we speak of 

 two structures as being homologous, it is not thereby implied that they 

 are completely so. Homology ends where fundamental resemblance ends. 

 This will become clear if we consider some examples. 



The classic example of homology is that afforded by the forelimbs of 

 air-breathing vertebrates. Whatever species we choose to examine — man, 

 horse, dog, elephant, seal, whale, bat, bird, lizard, dinosaur, or frog — the 

 skeleton and musculature of the forelimb prove to be fundamentally the 

 same. All these limbs are modifications of one architectural plan — that 



Fig. 24.3. Homologies between the bones of the forelimbs of mammals. {Redrawn by per- 

 mission from Schuchert and Dunbar, Outlines of Historical Geology, 4th ed., published by 

 John Wiley & Sons, Inc., 1941.) 



of the pentadactyl (five-fingered) limb. All these limbs are therefore 

 homologous. 



Let us now carry the analysis a little further. Where relationship is 

 close, as in dog and wolf, horse and ass, cattle and bison, crow and blue 

 jay, or whale and porpoise, the resemblance extends even to minor 

 details. Homology is essentially complete. In more distantly allied species, 

 such as wolf and whale, man and blue jay, or elephant and dinosaur, the 

 limbs are homologous in basic pattern but not in details of modification. 

 In other instances the resemblances between vertebrate forelimbs prove 

 to be compounded of homologous and analogous elements. An excellent 

 example is that furnished by the winged vertebrates. 



Three groups of vertebrates have the forelimb changed into a wing. In 

 birds the flight surface is formed of feathers (modified scales) attached to 

 an arm in which some of the digits are lost and the others are fused to form 

 a stiff supporting axis. In bats the wing is a sheet of skin stretched be- 

 tween four elongate, outspread fingers and attached to the body and hind 

 legs and usually to the tail. In the extinct flying reptiles, called pterosaurs, 



