660 



ORGANIC EVOLUTION 



giving a composite picture (Fig. 11-16). 

 Thus, the problem of receiving Hght has 

 been satisfactorily solved but in an entirely 

 different manner from that of the mollusk 

 or vertebrate. On the other hand, the like- 

 ness of the vertebrate and squid or cuttle- 

 fish eye (Fig. 26-5) is amazing. The fact 

 that two such distantly related groups of 

 animals could solve photoreception in al- 

 most identical ways seems incredible. Each 

 animal had different materials at hand to 

 build a photoreceptor and each built it 

 differently, but both came out with essen- 



evolved toward life in a particular environ- 

 ment; this is called convergent evolution. 

 The manner in which these two types of 

 evolution operate can best be understood 

 by following a specific structure or organ 

 through its evolution in several groups of 

 animals. A good example is the forefoot 

 of land vertebrates. 



The ancestral land vertebrate appendage 

 was a five-toed structure designed for sur- 

 face locomotion. As new environments be- 

 came available the forefoot underwent 

 divergent evolution, becoming highly modi- 



Fig. 26-5. There is a remarkable similarity between the molluscan (cuttlefish) and vertebrate 

 (human) eyes, yet they have evolved along two quite different pathways. This type of 

 evolution is called convergent evolution and is abundantly illustrated among animals. 



tially the same organ, varying only in the 

 placement of the sensory cells of the retina 

 (the rods and cones in the vertebrate 

 are directed away from the light source, 

 whereas in the molluscan eye they are re- 

 versed). The pathways by which these 

 eyes were formed is entirely different in the 

 two groups, which merely proves the earlier 

 contention that animals solve similar prob- 

 lems in different ways. 



Divergence and convergence in evolution 



As opportimities have presented them- 

 selves, animals have evolved in many direc- 

 tions with respect to certain organs in 

 order to take advantage of all possible en- 

 vironments. This is called divergent evolu- 

 tion or adaptive radiation. At some later 

 time some members of various groups have 



fied for locomotion in many ways. For ex- 

 ample, it became a single-toed leg in horses, 

 a five-toed clawed leg in cats, a flipper in 

 whales, an arm in man, and a bird's wing 

 (Fig. 26-6). This fundamental five-digited 

 structure has become an effective instru- 

 ment for locomotion in water, on land, and 

 in the air. This is the situation in which we 

 find vertebrates today. But long before ver- 

 tebrates reached their present state, ances- 

 tral forms, even after they became adapted 

 to land life, had the opportunity of taking 

 to the air which has some obvious advan- 

 tages over locomotion on land. Members of 

 three groups took advantage of this oppor- 

 tunity and underwent convergent evolu- 

 tion toward locomotion in the air. These 

 were: first, the reptiles (pterodactyls), then 

 the birds, and finally the mammals (bats) 



