OcTOBER 15, 1897.] 
force in certain structures, such as the teeth, 
which are not improved by individual use 
or exercise, as in structures which are so 
improved. A very large class of determi- 
nate variations in other stationary charac- 
ters, such as the inner parts of the skull, 
also remain unexplained. My study of teeth 
in a great many phyla of Mammalia in 
past times have convinced me that there 
are fundamental predispositions to vary in 
certain directions; that the evolution of 
the teeth is marked out beforehand by he- 
reditary influences which extend back hun- 
dreds of thousands of years. These pre- 
dispositions are aroused under certain ex- 
citing causes and the progress of teeth de- 
velopment takes a certain form converting 
into actuality what has hitherto been po- 
tentiality. 
Edward B. Poulton, M. A., F. R. S., 
Hope Professor of Zoology in the Univer- 
sity of Oxford, continued the discussion. 
He began by saying that it must be ad- 
mitted that the adaptation of the  indi- 
vidual to its environment during its own 
life-time possesses all the significance at- 
tributed to it by Professor Osborn, Pro- 
fessor Baldwin and Professor Lloyd Mor- 
gan. These authorities justly claim that the 
power of the individual to play a certain part 
in the struggle for life may constantly give 
a definite trend and direction to evolution, 
and that, although the results of a purely 
individual response to external forces are 
not hereditary, yet indirectly they may re- 
sult in the permanent addition of corre- 
sponding powers to the species, inasmuch 
as they may render possible the operation 
of natural selection in perpetuating and in- 
ereasing those inherent hereditary varia- 
tions which go further in the same direction 
than the powers which are confined to the 
individual. 
Professor Osborn’s metaphor in opening 
this discussion puts the matter quite clearly 
and will be at once accepted by all Dar- 
SCIENCE. 
585 
winians. If the human species were led by 
fear of enemies or want of food to adopt an 
arboreal life all the powers of purely indi- 
vidual adaptation would be at once em- 
ployed in this direction and would produce 
considerable individual effects. In fact, the 
adoption of such a mode of life would at 
first depend on the existence of such powers. 
In this way natural selection would be 
compelled to act along a certain path, and 
would be given time in which to produce 
hereditary changes in the direction of fit- 
ness for arboreal life. These changes would 
probably at first be chiefly functional, as 
Mr. Cunningham has argued (in the Preface 
to his Translation of Himer). On these 
principles we can understand the arboreal 
kangaroo (Dondrolagus) found in certain 
islands of the Malay Archipelago, which is 
apparently but slightly altered from the 
terrestrial forms found in Australia. Pro- 
fessor Osborn has alluded to the arbo- 
real habits said to have been lately ac- 
quired by Australian rabbits; these and 
the similar modifications in habits of West 
Indian rats are further examples of indi- 
vidual adaptive modification which may 
well become the starting point (in the sense 
implied above) of specific variation led by 
natural selection in the definite direction of 
more and more complete adjustment to the 
necessities of arboreal life. Although this 
conclusion seems to me to be clear and sound, 
and the principles involved seem to consti- 
tute a substantial gain in the attempt to un- 
derstand the motive forces by which the 
great process of organic evolution has been 
brought about, I cannot admit that the im- 
portance of natural selection is in any way 
diminished. I do not believe that these im- 
portant principles form any real compromise 
between the Lamarckian and Darwinian 
positions, in the sense of an equal surrender 
on either side and the adoption of an inter- 
mediate position. The surrender of the 
Lamarckian position seems to me complete, 
