
No. 425.] LAW OF ADAPTIVE RADIATION. 361 
so far as we know, into the Oligocene genus Co/odon, the three- 
toed type with extraordinarily elongate digits. So far as we 
know, this light-limbed series is found both in America and 
Europe, while the heavy-limbed Lophiodontinz are found only 
in Europe. 
Since the above was written the titanotheres have been 
more carefully examined by the writer (Osborn, '02b), and, like 
the rhinoceroses, they are found to subdivide into four con- 
temporaneous phyla distinguished chiefly by dolichocephaly 
and brachycephaly and by relatively long and short limbs, 
thus affording another conspicuous illustration of this daw of 
local adaptive radiation. 
IV. RADIATION AND CORRELATION OF STRUCTURE. 
In the careful consideration of adaptive radiation from cer- 
tain stem types is to be found the true significance of Cuvier's 
law of correlation as modified by the — to him — unknown prin- 
ciple of evolution. Referring to the diagrams, Fig. 4, two 
important principles are brought out: First, practically all the 
adaptations known among mammals have arisen by combina- 
tions of divergence independently pursued in the limbs and 
teeth; for example, an herbivorous tooth type may combine 
with a terrestrial, arboreal, or volant limb type, according as 
the search for plant food is on the earth, in the trees, or in 
the air. Although every imaginable combination (e.g., aquatic 
limbs, myrmecophagous dentition) cannot be realized, yet these 
combinations have been multiplied almost ad infinitum and 
constitute the fatal defect of Cuvier's law as he conceived it. 
As tested by a single case, the Eocene monkeys of the family 
Notharctide acquired teeth exactly homoplastic with those of 
Eocene horses, but the former were provided with arboreal, 
the latter with terrestrial, limb types. Second, correlation of 
limb and tooth structure in a given group is further condi- 
tioned by the particular combination and degree of specializa- 
tion of limbs and teeth which the radiation originates with. 
For example, the primitive placentals combined tritubercular 
insectivorous teeth with a generalized or probably terrestrial 
