GENERAL ZOOLOGY 



each with its own direction. In the sequence leading to modern elephants, 

 however, the following changes are notable: (1) the snout becomes elongated 

 to form the trunk; (2) the jaws become shortened; (3) the teeth become modi- 

 fied and complicated and undergo marked specialization, both in structure 

 and in the manner of their succession; (4) the shape of the skull changes 

 remarkably; and (5) there is a great increase in the size of the body and of 

 the brain. Members of the elephant family flourished and ranged widely in 

 Europe and North America but disappeared from both continents at the close 

 of the Pleistocene. The mastodons, the only type which reached South 

 America, persisted until recent times but eventually disappeared. Today, the 

 only survivors of the elephant family are the modern African elephants and 

 those native to Southeast Asia. 



Synthetic Types. Critics of the general idea of organic evolution often 

 raise the objection that the connecting links which should exist between 

 various types of animals are not to be found in the modern fauna. It should 

 be clear, however, that such links, or synthetic types, are not to be sought 

 among existing animals but among fossils. The connecting links between 

 different types of modern animals were their common ancestors, which have 

 disappeared through extinction. The connections between modern horses, 

 zebras, and wild asses were the extinct members of the horse family ancestral 

 to all the modern types. The links between the Asiatic and African elephants, 

 and between the existing south American guanacos and the camels of the Old 

 World, were likewise the extinct common ancestors. The links between men 

 and apes were the extinct prehuman primates, the earliest of which were 

 more ape-like than man-like. 



Connections between larger groups are not wanting; they are clearly 

 represented by the fossil remains of forms transitional between amphibians 

 and reptiles, between reptiles and birds, and between reptiles and mammals, 

 as described in Chapter 18. It should not be expected that in these instances 

 one single animal will show all the transitional structures. Striking ex- 

 amples of such transitional forms are the primitive birds, notably Archaeopteryx, 

 which show many reptilian characteristics (see Fig. 18.25, p. 575). The 

 development of a bird resembles that of a reptile, and the structure of the 

 adults is more similar than would be supposed from casual examination. 

 Archaeopteryx is perhaps as nearly a synthetic type as we could hope to 

 find, in view of the imperfections of the fossil record. 



In addition to the fossil remains of such forms, animals that are some- 

 times called "living fossils" still exist. The duckbill, Ormthorhynchus (see 

 Fig. 18.30, p. 581), is an egg-laying mammal which may be regarded as a 

 survivor not far removed, in its structural and reproductive features, from 

 the egg-laying reptilian stock that gave rise to the mammals. The existing 

 lungfishes may similarly be interpreted as links between aquatic and terrestrial 

 vertebrates; these might have become extinct, like Archaeopteryx, but instead 

 they have survived and lend supp>ort to the theory of evolution. The lobe- 

 finned fishes (see p. 557) were believed to have been long extinct and were 



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