398 GENERAL PRINCIPLES 



provided with a complete series of teeth. Some of the birds 

 of the Cretaceous period were much more like modern birds, 

 but still had teeth on certain parts of the jaws. The Archaeop- 

 teryx was almost as much reptile as bird, and even the later 

 types presented many decided reptilian characters. 



807. The most complete series of mammalian fossils are those 

 which show the genesis of the modern horse. The earliest 

 mammals were pentadactyl (Phenacodus) . The earliest horse- 

 like form had four toes on the fore foot and three on the hind 

 foot. From this we see the number of toes gradually reduced, 

 until in the modern horse there is only one functional digit and 

 small rudiments of the second and fourth. The first digit (I) 

 was the first to disappear and then the fifth (V). These were 

 then followed by the second and fourth. 



808. Geological evidence concerning the history of the 

 development of the vegetable kingdom is much like that for 

 animals. The Cryptogams existed in great profusion long be- 

 fore the seed-bearing plants appeared, and the Gymnosperms 

 preceded the Angiosperms. 



809. The Ontogenetic Series. The differences between indi- 

 viduals are the greatest when the individuals are mature. The 

 young, the late embryos and the earlier embryonic stages are 

 successively more and more alike, and differences finally vanish 

 in the egg. Hence all metazoa start from the same level. In 

 fact, the differences which exist up to the end of gastrulation 

 are of secondary importance and have no relation to the sys- 

 tematic rank of the developing organism. All metazoa are, 

 therefore, in a real sense alike up to the end of gastrulation. 



8 10. Suppose an observer entirely unacquainted with the 

 characteristics of eggs were given a series of eggs representing, 

 we will say, Ccelenterates, Annelids, the lancelet, a fish, a bird 

 and a rabbit. In the eggs themselves the observer would find 

 no means of determining the class to which they belonged. If 

 now these eggs each passed through its appropriate develop- 



