566 The American Naturalist. [July, 
influence. Neither, in my opinion, directly supports the theory — 
of transmission of acquired characters, for they do not prove 
that normal changes in the body-cells directly react upon the 
germ-cells; they all show that the typical hereditary develop- 
ment of single organs may be diverted by living forces which have 
no direct connection with them according to our present know- 
ledge. 
What the nature of these forces is I will not undertake to 
say, but I believe we must admit the existence of some 
unknown force, or rather of some unknown relations between 
the body-cells and germ-cells. 
A year ago, recognizing fully the difficulty of advancing 
any theory of heredity which would explain the transmission 
of acquired characters, I came to the following result: “ It 
follows as an unprejudiced conclusion from our present evi- 
dence that upon Weismann’s principle we can explain inher- 
itance but not evolution, while with Lamarck’s principle and 
Darwin’s selection principle we can explain evolution, but not, 
at present, inheritance. Disprove Lamarck’s principle and we 
must assume that there is some third factor in evolution of 
which we are now ignorant. g 
In this connection it is interesting to quote again from my 
colleague, Professor E. B. Wilson. He writes that the tendency 
in Germany at present is to turn from speculation to empiri- 
cism, and this is due partly “to the feeling that the recent 
wonderful advances in our knowledge of cell phenomena have 
enormously increased the difficulties of a purely mechanico- 
physical explanation of vital phenomena. In fact, it seems 
that the tendency is to turn back in the direction of the vital- 
force conception. . . As Boveri said to me recently, ‘ Es 
gibt zu viel Vorstand in aor Natur um eine rein mechanische 
Erklärung der Sache zu erméglichen.’” 
In the final lecture we turn to the forces exhibited in the 
germ-cells. 
Ai sant Sic oa) oa 
Sail cy Sone eee ae Nae 
