A DEFINITION OF EVOLUTION 



Later, amphibians and reptiles appear in the fossil record, and birds and 

 mammals finally appear quite late. Thus the simplest animals appear in 

 the most remote geological eras, while the most complex appear late in 

 geological history. In most major groups (order, class, and phylum), there 

 is marked change from one geological period to the next, but always a 

 particular fauna resembles that of another period near it in time more 

 closely than it does that of any other period remote from it in time. 

 Finally, the fossils of recent organisms blend into our present living flora 

 and fauna, with often the same genera and even the same species being 

 represented. 



CATASTROPHISM VERSUS EVOLUTION 



Before the time of Darwin, it was customary to explain these facts by 

 assuming that all life had been destroyed by catastrophes from time to 

 time, each catastrophe being followed bv a new creation. As more and 

 more paleontological information accumulated, however, it became appar- 

 ent that the number of catastrophes necessary to account for the known 

 succession of floras and faunas was absurdlv large, and further that ex- 

 tinction of difterent contemporaneous groups was not simultaneous, as 

 would be necessary under the theory of catastrophism. Under Darwin's 

 theory, it was not necessary to assume any catastrophes. Species simply 

 change continuously under the influence of natural selection. The inevi- 

 table result is a changed aspect of the flora and fauna from one period 

 to the next, with the difference increasinii throughout time. This does not 

 require that the rate of change in difterent groups, or in different members 

 of the same group, be the same. 



EVOLUTION OF THE HORSE 



Perhaps tlie most thoroughlv known phylogenv of anv vertebrate is that 

 of the horse. The history of the horse, as understood today, co\'ers a period 

 of about 60,()()(),()()() years, beginning in the Eocene Period, and involving 

 twenty genera and a much larger niunber of species, although most of 

 these are not believed to be in the direct line of descent. The oldest mem- 

 bers of the superfamilv EfjuoidcM were the eight genera of the family 

 PaU'otheriidae, but these arc usually not taken into account in discussions 

 of horse phylogeny, partly because of uncertainty regarding their relation- 

 ship to more advanced equoids, but also because they show as nuich 

 relationship, or more, to other mammals as to horses. Their "hooves, " for 

 example, are quite claw-like. 



The usual starting point of horse plnlogeni(\s is with I lijidcollwriuin 

 (including Eoltippii.s of many public-ations), the oldest known nuMuber of 

 the family Equidae. This little animal stood less than a loot higli at l\\c 

 shoulder, and browsed in forest underbrush (known Irom the character 

 of the teeth)- Th(> teeth were fortv-four in niunber, and the cheek teeth 

 were only moderatelv sp(X'iali/.(xl for grinding. Th(> Iront fec^t had four 

 toes and a splint, while the hind feet had three toes and two splints (rep- 



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