
354 THE AMERICAN NATURALIST. [VoL. XXXVI. 
principles of homoplasy, parallelism, and convergence, which, 
as shown in the last number of the Naturalist, are by no 
means synonymous terms or identical processes. The alto- 
gether similar law of /oca/ adaptive radiation or incipient diver- 
gence on a smaller scale in a single locality may now be more 
clearly developed. 
This idea of radiation becomes a means of interpretation, 
and a way of imagining the relations of extinct and living 
faunæ. As perceived by Cope, it applies both on a small 
and on a vast scale. 
I. GENERAL RADIATION. 
In the * Rise of the Mammalia” ('93, pp. 30-33) the ancient 
Mesozoic (Meseutheria) and modern Cenozoic (Ceneutheria) 
differentiation of the placentals (Eutheria) was spoken of as 
follows : 
The Puerco is essentially an archaic fauna, and is to be regarded as the 
climax of the first period of placental differentiation, a culmination of the 
first attempts of nature to establish insectivorous, carnivorous, and her- 
bivorous groups. These attempts began in the Cretaceous, and some of 
the types thus produced died out in the Puerco, some in the Wasatch and 
Bridger ; only a few flesh-eaters survive to the Miocene. It is most impor- 
tant to grasp clearly the idea of this functional radiation in all directions 
of this old Puerco fauna, resulting in forms like the modern insectivores, 
rodents, bears, dogs and cats, monkeys, sloths, bunodont and selenodont 
ungulates, and lophodont ungulates. This was an independent radiation 
of placentals, like the Australian radiation of marsupials. 
Some of the least specialized spurs of this radiation appear to have sur- 
vived and become the centers of the second or mid-Tertiary radiation, from 
which our modern fauna has evolved. Yet we have not in a single case 
succeeded in tracing the direct connection. To sum up, we find on the 
North American continent evidence of the rise and decline and disappear- 
ance of monotremes and marsupials, and two great periods of placental 
radiation, 
ation, the ancient radiation beginning in the Mesozoic, reaching a 
climax in the Puerco and unknown post-Puerco, and sending its spurs 
into the higher Tertiary, and the modern radiation reaching its climax in 
the Miocene, and sending down to us our existing types. 
1 This statement has been modified by iibetquent discovery. 


