Progress in Mammalian Palaeontology. 
203 
Evolution in part determinate. — As regards the modes 
and factors of evolution 2 , the continuous stages of evolution 
which we are securing among the horses, camels, rhinocer- 
oses, and many other families, afford opportunities which 
have never been afforded before. We are with adaptive 
characters from their birth or genesis, through their prime, 
into their decline and death. Through this unique oppor- 
tunity for observation has been confirmed a view of evolu- 
tion long shared by most if not all palaeontologists, verte- 
brate and invertebrate, but naturally not understood or 
PRINCIPAL FORMATIONS IN WHICH FOSSIL MAMM. 
ARE FOUND IN WESTERN UNITED STATES 
CHARACTERISTIC 
European rvmnxu 
Equus lUnorm 
Hippa rion 
-Inchithenum 
Mastodon 
Dice rathe ri urn 
/ialitherium 
f&ladhmum 
j 
Lophiodon 
Coryphodon 
Wrcuvwn 
CORRESP EUROP 
CORnATIONS 
roots! ecus wo/cm 
'L0»fiNOSUH«EURE 
KtRMI 
0«'vf SI AL6AN 
SAN SAM 
sables or corl 
5_^B«ri-i MIOCENE 
ST CERANO LE PlA 
SASLCS Ot FOWTAlNCBt EAU 
RON 20* 
CESSfRAS 
CALC GROS SUP 
CE.P.*A"S'EN 
CACC CROS OE "ONS 
CftEAT PLAINS SECTION 
CHARACTERISTIC 
AMERiCaA) WA1M«LS 
Bison 
PLEISTOCENE 
PLIOCENE 
Elephaj 
OLIGOCENE 
UTAH SECTION 
EOCENE 
|^r.tLll0ffeeE5^v "5£&g * ^%-lCoryphodon 
MONTANA SECTION -1^. £l — ■ -' \jj.r^£^^ S^PEEH?. J 
-5-Jp^ftOO^ . Euprotog, 
^^B&&€TiPolumajlodrjn 
tLARAME 
- ■.-. ct^nrop*-. 
Equus 
Pliauchcnia 
Pltohippus 
Tnloptiodon 
Prvtohippus 
Aphclops 
Anchithcnum 
Ccenop us 
Protohippui 
Titanothertum 
.'JJinojaurio) 
FIG 1. 
Most recent geological subdivision of the American Tertiary. 
Showing that the successive sections in Montana, New Mexico, Wyo- 
ming, Utah, Oregon, and the Greal Plains afford a complete history ol 
tin- Tertiary, homotaxial with that afforded by the corresponding Euro- 
pean formations. 
shared by other zoologists because of the essentially differ- 
ent nature of evidence. 1 refer especially to the theory of 
2 Scott, W. B. On the Mode of Evolution in the Mammalia and on 
some of the Factors m the Evolution of the Mammalia. Jour. Mmplinl. 
vol. v, 1891, No. :;. pp. 361-378, 378-4u2. 
