822 THE AMERICAN NATURALIST. {VoL XXXII. 
It is taken for granted that readers are familiar with the 
orthodox, or common working hypothesis, of modern natural- 
ists in regard to these matters; but, in order that we may 
be thinking alike, let me mention a few particulars to which 
special attention is directed. 
(A) It is assumed to be the accepted belief that organisms 
acquire their distinctive characters by the processes of natural 
growth and development. 
(B) That it is the accepted belief that those characters 
which in a particular organism are /#e the characters of its 
parents and ancestors are to be explained on the hypothesis of 
heredity, i.e., that organisms naturally reproduce offspring like 
themselves. 
(C) That it is the accepted belief that the characters which 
are unlike those of the parents are explained on the hypothesis 
of variation, i.e., that organisms differ slightly or vary natu- 
rally from their immediate ancestors. 
It will be noticed that the term organisms in the second 
statement refers to the parents in the case; in the third state- 
ment it refers to the developing offspring. I understand this 
to be the accepted view, 7.¢., that it is assumed that the causa- 
tive factor determining the hereditary reproduction of /2ke 
characters is associated with the parent; and, on the other 
hand, that the causative factor of diversity is associated with 
the individual varying in response to diversity in the condi- 
tions under which it develops. Or, to put this whole idea 
definitely as a separate proposition, 
(D) It is the orthodox hypothesis, regarding this question, 
that an organism arising under conditions entirely similar to 
those of its immediate ancestors, would not vary from them, but 
would develop in perfect facsimile to them; and, therefore, that 
variation ts incident to heterogeneity of environment. 
Is all this true? Or is not the very converse of it true? 
In opening the discussion of this question, let me refer bi 
the opinions of the founders of the evolution theory on this 
point. 
In the year 1861 Darwin wrote to Thomas Davidson: “ My 
greatest trouble is, not being able to weigh the direct effects 
