7° READINGS IN EVOLUTION, 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. 
g. 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.—Ep.] 
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. 
