
No. 425] LAW OF ADAPTIVE RADIATION. 355 
In the * Origin of the Mammals ” ('99, p. 92) the idea was 
developed as follows : 
To guide our speculation in the unknown pre-Tertiary period, we may 
gather certain positive principles from the known evolution of the Tertiary 
Mammalia. First, we know that adaptive radiation, characteristic of all 
vertebrates, and beautifully illustrated among Reptilia, is in a very high 
degree distinctive of Mammalia, because of their superior plasticity. 
The focal-types, or most primitive forms of the radiations, I-IV, «vere 
certainly small, terrestrial, clawed, insectivorous or omnivorous forms. 
It is noteworthy that in the evolution of each radiation, so far as we know 
at present, land types and organs are invariably primitive, and water types 
and organs are secondary, exactly as we find among the Reptilia. In fact, 
we have not found a single instance in which a mammal or reptile series 
is known to be transformed from a water into a land type; it is always the 
reverse. There is certainly no evidence for a cetoid (Albrecht) stem of 
the Mammals. Again, it is obvious that neither carnivorous nor herbivo- 
rous types with highly specialized or reduced teeth and feet can be so 
central as insectivorous and omnivorous types. In fact, the Insectivores 
among Placentals, and Opossums among Marsupials, are the only animals 
which have preserved the dental prototype close to that of the Promammal. 
The radiations spoken of in this essay were: 
I. Marsupial Radiation of Australia (Meteutheria). 
Il. Tertiary Placental Radiation of the Northern Hemisphere, i.e., 
North America, Asia, and Europe (Ceneu eria). . 
Ill. Tertiary Placental Radiation of South A merica (Ceneutheria). 
IV. Cretaceous Placental Radiation of North America (Meseutheria). 
V. Jurassic Radiation of Placentals and Marsupials. 
In a subsequent paper, * The Geological and Faunal Rela- 
tions of Europe and America during the Tertiary Period, and 
the Theory of the Successive Invasions of an African Fauna " 
(00), the subject was further developed as follows : 
Now it is a well-known principle of zoólogical evolution that an isolated 
region, if large and sufficiently varied in its topography, soil, climate, and 
vegetation, will give rise to a diversified fauna according to the law of 
adaptive radiation from primitive and central types. Branches will spring 
off in all directions to take advantage of every possible opportunity of 
securing food. The modifications which animals undergo in this adap- 
tive radiation are largely of mechanical nature, they are limited in number 

