70 READINGS IX E\'OLUTION, GENETICS, AND EUGENICS 



2. The animals and plants of each geologic stratum are at least 

 generically different from those of any other stratum, though belonging 

 in some cases to the same families or orders. 



3. The animals and plants of the oldest (lowest) geologic strata 

 represent all of the existing phyla, except the Chordata, but the 

 representatives of the various ' phyla are relatively generalized as 

 compared with the existing types. 



4. The animals and plants of the newest (highest) geologic strata 

 are most like those of the present and help to link the present with 

 the past. 



5. There is, in general, a gradual progression toward higher types 

 as one proceeds from the lower to the higher strata. 



6. Many groups of animals and plants reached the climax of 

 specialization at relatively early geologic periods and became extinct. 



7. Only the less specialized relatives of the most highly specialized 

 types survived to become the progenitors of the modern representa- 

 tives of their group. 



8. It is very common to find a new group arising near the end of 

 some geologic period during which vast climatic changes were taking 

 place. Such an incipient group almost regularly becomes the domi- 

 nant group of the next period, because it developed under the 

 changed conditions which ushered in the new, period and was therefore 

 especially favored by the new environment. 



9. The evolution of the vertebrate classes is more satisfactorily 

 shown than that of any other group, probably because they represent 

 the latest phylum to evolve, and most of their history coincides with 

 the period within which fossils are known. 



10. Most of the invertebrate phyla had already undergone more 

 than half of their evolution at the time when the earliest fossil remains 

 were deposited. — Ed.] 



FOSSIL PEDIGREES OF SOME WELL-KNOWN VERTEBRATES 



PEDIGREE OF THE HORSE 



[Of all fossil pedigrees that of the horse is most often mentioned in 

 evolutionary literature. The main facts have been known for about 

 forty years, and there is a rather general consensus of opinion as to the 

 history as a whole. It appears practically certain that the horse 

 family (Equidae) arose from a group of primitive five-toed ungulates 

 or hoofed mammals called Condylarthra that lived in Eocene times. 



