GENERAL ZOOLOGY 



It is frequently possible to explain the peculiarities of an entire fauna, 

 the animal population of a particular locality, by the extension of a similar 

 line of reasoning. Geologically speaking, there are two types of marine 

 islands: continental islands, such as Long Island and the British Isles, and 

 oceanic islands, such as the Azores, Bermuda, and Hawaii. Continental 

 islands are near continents, to which they were probably attached in past 

 times; oceanic islands have appeared in the ocean without previous connec- 

 tion with any continent, or they may be the surviving mountain tops of former 

 land masses. The faunas of these two types of islands bear out the theories 

 of the geologic origins of the islands, and of the origins of the faunas by 

 evolutionary processes. The native fauna of the British Isles resembles that 

 of Northwestern Europe, with which they were evidently formerly connected. 

 The native fauna of Hawaii, in contrast, is an odd mixture of forms and is 

 unlike that of any continent. Presumably, as discussed in Chapter 19, the 

 faunas of such oceanic islands have developed by adaptive radiation from 

 chance immigrants which reached the islands on floating objects or otherwise. 

 One of the peculiarities of Hawaii is that there are no native amphibians; 

 these animals cannot tolerate exposure to salt water and have therefore been 

 unable to reach the isolated islands. The continent of Australia has a fauna 

 very different from that of Asia, because the two have been so long separated 

 (see Fig. 19.5, p. 609). Eurasia, Africa, and North America have similar 

 mammalian faunas because of former migration routes across land bridges 

 from Alaska and by way of Greenland. 



EVIDENCE FROM MORPHOLOGY 



Comparative Anatomy of Adults. Whether we examine the broader 

 features of anatomy in the several phyla of animals or the structure 

 typical of a single group, such as vertebrates, we find everywhere facts 

 that are most logically interpreted from the standpoint of evolution. In 

 the vertebrates, two pairs of limbs, a trunk, head, tail, and various internal 

 organs are always laid down according to a similar general plan, but with 

 special modifications in relation to the mode of life. Therefore, it is pos- 

 sible to construct a plan which is characteristic of vertebrates in general. 

 Similarly, as indicated in preceding chapters, a characteristic plan can be 

 made for animals in each of the phyla; there is a coelenterate plan, an annelid 

 plan, an arthropod plan, and so on. More specific resemblances in the cor- 

 responding parts of the body are revealed when comparisons are made among 

 animals of a single class. Despite their differences, the flipper of a whale, 

 the wing of a bat, bird, or pterosaur, the fore limb of a horse, and the arm of 

 a man all show the same general plan of structure. Such similarities in funda- 

 mental design, despite special adaptive modifications for specific functions, 

 illustrate the principle of homology; examples are numerous in every phylum 

 of the Animal Kingdom. 



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