200 ANNALS Ni:\\ YORK ACADEMY OF SCIENCES 



spectively, in the western Plains, and tlie degree of consolidation of the 

 matrix is the same. We have in the West two fossil if oroiis formations, 

 the Bridger (Eocene) and John Day (Oligocene), which arc. like the 

 Santa Cruz, composed of an andesitic volcanic ash, and simihir ash strata 

 are found in different levels of our Western Miocene formations. Now, 

 the Santa Cruz matrix and fossils are vqry much less consolidated or 

 thoroughly petrified than the Bridger and 'decidedly less so than the 

 John Day, while they agree very well with the volcanic ash beds in the 

 middle and u])per Miocene. As there is no reason to suppose that the 

 rock-making processes woi-k at a different rate in different continents, 

 this evidence is entitled to some consideration. On similar grounds, the 

 Pampean fossils would be referred to middle Pleistocene, and the few 

 fossils that I have seen from Monte Hermoso agree best with Pliocene 

 fossil mammals from North America. I should place no weight on this 

 kind of evidence except when, as in the present instance, the climatic 

 conditions and the origin and method of deposition of the formations are 

 substantially similar. 



The foregoing digression is somewhat outside the limits of this dis- 

 cussion. It appears, however, to be necessary to show briefly the reasons 

 on which the age assigned to the South American mammalian faunae are 

 based. It might, indeed, be logically objected that these correlations 

 are based on the northern origin and migration of certain phyla and 

 cannot, therefore, be used in support of the theories here advocated. But 

 the phyla on which the demonstration rests are so universally admitted 

 to have arisen in the north, and the evidence that they did so is so com- 

 plete and conclusive, that there is no reasonable alternate to accepting 

 them as such. And if so, the correlations of South American faunae 

 must be approximately as here stated, a conclusion supported by the 

 wholly independent evidence of the degree of consolidation of the fonna- 

 tion and of petrifaction of the fossils contained. 



Cp:nti:rs of Dispersal 



W'liclhci- the evolution of a race be regarded as conditioned wholly by 

 the extenuil en\ironment or as partly or chiefly dependent upon (un- 

 known) intrinsic factors, it is admitted by everyone that it did not 

 appear and progress simultaneously and (''(jiio pede over the whole sur- 

 face of the earth, or even over the whole area of a great continent. The 

 successive steps in the progress must appear first in some comparatively 

 limited region, and from that region the new forms must spread out, 

 displacing the old and driving them before them into more distant 



