302 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



when the Cretaceous series was found. Six or seven Cretaceous 

 faunae have been discovered. Then we have the Bridger fauna in 

 1870, the Wasatch fauna in 1874. Next we have, in 1877, the 

 Equus beds and the fauna which they embrace, part of which was 

 also found in 1878. The Permian fauna, which is one of the last, 

 in 1879 ; and the latest, the Puerco, which gives the oldest and 

 ancestral types of the modern forms of Mammalia, was only found 

 in 1881. When I first commenced the study of this subject, about 

 1860, there were perhaps two hundred and fifty species known. 

 There are now somewhere above 1,000, and we are augmenting 

 them all the time. I have found many myself : if they were dis- 

 tributed through the days of the year, I think in some months I 

 should have had several every day. You see then that the acces- 

 sions to knowledge which are constantly being made make it un- 

 safe to indulge in any prophecies ; as, for instance, that, because 

 such and such things have not been found, therefore they can not 

 be ; for we find such and such things really have been and really 

 are discovered. 



The successive changes that we have in the Mammalia have 

 taken place in the limbs, feet, teeth, and brain, and the vertebral 

 column. The parts which present us with the greatest numbers 

 of variations are those in which many parts are concerned, as in 

 the limbs and feet. In the Lower Eocene (Puerco), the toes were 

 5-5. In the Loup Fork fauna, some possess toes but 1-1. Prior 

 to this period no such reduction was known, although in the Loup 

 Fork fauna a very few species remained 5-5. Through this en- 

 tire series we have transitions steady and constant, from 5-5, to 4- 

 5, to 4-4, to 4-3, to 3-3, to 2-2, to 1-1. In the Puerco period there 

 was not a single mammal of any kind which had a good ankle-joint ; 

 which had an ankle-joint constructed as ankle-joints ought to be, 

 with tongue and groove. The model ankle-joint is a tongue-and- 

 groove arrangement. In this period they were nearly all perfectly 

 flat. As time passes on we get them more and more grooved, un- 

 til in the Loup Fork fauna and the White Eiver fauna they are near- 

 ly all grooved. The soles of the feet, in the Puerco fauna, are 

 all flat ; but in the Loup Fork fauna the soles of the feet are in 

 the air, and the toes only are applied to the ground, with the excep- 

 tion of the line of monkeys, in which the feet have not become erect 

 on the toes, and the elephant, in which the feet are nearly flat also, 

 and the line of bears, where they are also flat. As regards the 

 angulation between the small bones of the palm and of the sole, 



