608 Validity of the Doctrine of Mutation. 
consists merely in an oscillation around a given point of 
equilibrium ; whilst the formation of new species necessi- 
tates the attainment of a new equilibrium. Especially in 
America has this view made great progress, as has been 
described by CONN in an admirable exposition in his new 
work on evolution. 1 
Of the numerous writings which \ve owe to WAL- 
LACE'S school, I shall only mention here that on Natural 
Selection and Tropical Nature, of this author (1895), 
and PLATE'S critical exposition of WALLACE'S theory. 2 
Although PLATE, at the conclusion of his clear and per- 
ceptive critique of the questions which pertain to this 
point, declares in favor of the theory of selection, his 
work, of all those of which I have made use, gives me the 
impression of the greatest objectivity; and I am con- 
vinced that the gulf between his views and those I hold 
will surely be bridged over some day. Therefore I refer 
the reader to his book for a closer study of these ques- 
tions, and shall confine myself here to a few points which 
stand in the closest relation to those discussed above. 
1. The significance of fluctuating variability is very 
limited, whereas the explanation of adaptations demands 
almost unlimited variability. In earlier days when the 
law of QUETELET was only known to apply to anthro- 
pology, almost all the changes of plants and animals 
were considered to be the consequences of ordinary varia- 
bility, but now this is shown to be governed by laws which 
largely curtail its importance. In the first part of the 
first volume this theme has been discussed in detail, and 
I may here simply refer the reader to those chapters. 
1 H. W. CONN, The Method of Evolution, New York, 1900, p. 132. 
2 L. PLATE, Ueber Bedcutung and Tragzvcite des Danvin'schcn 
Selectionsprincips, Leipsic, 1900. A very complete bibliographical 
list will be found on pp. 145-153 of this book. 
