VIII. 



THE EELATION OF MAN TO THE TERTIARY 



MAMMALIA.* 



Ii^" order to prove the affirmative of a doctrine of evolution by 

 descent of the existing types of living beings, two propositions 



must be established. The first, 

 that a relation of orderly succes- 

 sion of structure exists, which 

 corresponds with a succession in 

 time. Second, that the terms 

 (species, genus, etc.) of this suc- 

 cession actually display transitions 

 or connection by intermediate 

 forms, whether observed to arise 

 in descent, or to be of such vari- 



Fia. AA.-Periptychm rhabdodon part ^^^^ character aS to admit of nO 

 of posterior foot, two thirds natural size ; • ^ i • • • 



b, astragalus from above, showing flat other explanation of their origin 



face; e, metatarsals, the first lost, show- than that of deSCCnt.. 



ing plantigrade foot. Original; from j^ ^|-^g g^i^j ^f paleontology it 



Puerco Epoch of New Mexico. . ., -i i j t 7 i 



IS quite possible to demonstrate 

 the first of these propositions, while the proof of the second is 

 necessarily restricted to the observation of variations and the dis- 

 covery of connecting forms which destroy the supposed definitions 

 of species, genera, etc. The conditions are more favorable for the 

 investigation of animals of the higher types than of those of the 

 lower. Their late origin insures to us the opportunity of discovery 

 of their ancestry far more certainly than in the case of the lower, 

 whose beginnings are lost in the remote past, and belong to periods 

 whose deposits have undergone physical changes, or have been en- 

 tirely removed and redeposited elsewhere, thus insuring the de- 

 struction of the fossil remains once contained in them. The series 

 of the tertiary Mammalia is becoming more complete through the 



* Read before the American Association for the Advancement of Science, at 

 Detroit, 1875, under another title. 



