216 The American Naturalist. [March, 
of definite lines of Variation. What evidence has been advanced 
for the initial but all essential assumption that, for example, a tiny 
adaptive cusp is a factor in survival, while its tiny inadaptive fellow 
is a factor in extinction? not to mention the succeeding assump- 
tions which overwhelm us when we seek to derive definite adapta- 
tion from indefinite variations. 
The Lamarckian principle furnishes us with an explanation of 
the observed phenomena of simultaneous progressive adaptation 
in most of those parts which it affects, including Correlation and 
Parallelism. It cannot bé said at present to explain a// the phe- 
nomena within its sphere; we must explain these phenomena, or 
abandon the principle. 
It follows as an unprejudiced conclusion from our present 
evidence that upon Weismann’s principle we can explain Inheri- 
tance, but not Evolution, while with Lamarck’s principle and 
Darwin’s Selection principle we can explain Evolution, but not, 
at present, Inheritance. Disprové Lamarck’s principle, and we 
must assume that there is some third factor in Evolution of 
which we are now ignorant. 
