646 



ORGANIC EVOLUTION 



areas so far distant today? To account for 

 this one can consult the fossil records. It 

 has been found that the ancestor of these 

 two species roamed over not only their 

 present habitats but also Europe and North 

 America, which lie between them. Appar- 

 ently, relatives of the present living mem- 

 bers of the camel family died out in the 

 intervening regions. Similarly, the only 

 alligators in the world exist in southeastern 

 United States and the Yangtse River in 

 China. Fossil alligators have been found 

 in North America and Europe, which ac- 

 counts, then, for the present location of 

 living alligators. Just why they died out 

 over the large intermediate areas is con- 

 jectural. 



The theory of evolution is the most logi- 

 cal explanation for the distribution of ani- 

 mals over the earth. We may rest assured 

 that if and when a better explanation pre- 

 sents itself scientists will be the first to 

 recognize and accept it. 



RATES OF EVOLUTION 



From the foregoing discussion it is ob- 

 vious that evolution has gone on at dif- 

 ferent rates at different times, bringing 

 about radical changes in some groups 

 whereas in others little or no change at all 

 has occurred over millions of years. For 

 example, the chambered nautilus (Fig. 25- 

 19 ) evolved rapidly to a certain point after 

 which, for the past 75 million years, it has 

 not changed. During the same period of 

 time the horse has come all the way from 

 a many-toed, shy little animal to the huge 

 beast we see today. The nautilus evolved 

 very rapidly from Silurian times, reaching a 

 peak in the Jurassic and then coming to a 

 standstill in the Tertiary with no change 

 from that point up to the present. All of its 

 relatives died out in this early period and 

 it was only by some peculiar circumstance 

 that this "living fossil" has survived to the 

 present time. The horse, on the other hand. 



has shown a progressive change up to the 

 present and is still changing (Fig. 26-4). 



Even if the whole animal seems to have 

 evolved steadily, a careful examination of 

 its various structural aspects shows that it 

 has undergone change at varying rates. For 

 instance, among the early Pliocene horses 

 some show marked changes in both teeth 

 and feet over the ancestral Miocene horse, 

 others show changes in teeth but not feet, 

 and still others show no change in feet or 

 teeth. This indicates that the teeth and feet 

 have chano;ed at different rates and at dif- 

 ferent times in the various groups of horses. 

 Similar instances can be found in other 

 groups of animals. It is apparent that struc- 

 tural evolution has gone ahead erratically 

 with bursts of speed and sudden pauses for 

 reasons which we do not know at present 

 and may never know. 



Another way of studying the rates of evo- 

 lution is to determine the number of dif- 

 ferent kinds of animals produced, that is, 

 the number of new species, new genera, 

 and even larger categories that came into 

 existence in a specified period of time. 

 Using as factors the number of genera of 

 animals in a family and the length of time 

 known, it is possible to compute how many 

 years are required to produce a new genus. 

 This has been done for the familiar horse 

 lineage and it has been estimated that a 

 new genus appeared about every 7,500,000 

 years. Estimates for other hoofed mammals 

 seem to be about the same during that pe- 

 riod, although this generalization does not 

 hold for all animals, even mammals. The 

 opossum, for example, has changed very 

 little if any since the Cretaceous, over 80 

 million years ago, and at tliat time it was 

 the most advanced of all animals. This is 

 certainly substantial evidence that some 

 animals have evolved much more rapidly 

 than others, and also that the rate may vary 

 greatly at different times within a small 

 group of animals. 



Considerable data, brought together by 

 G. G. Simpson, show the rate at which 



