224 
j. Bret land Farmer. 
the outcome of chemical change, and I can find no other satisfactory 
hypothesis that will account for the facts, it would seem difficult to 
escape from some such conclusion, but whether it be correct or not it 
could probably be submitted to experimental test in a favourable 
case. 
The relation of mechanism and stimuli may be considered in 
yet another connection, but I shall say very little about this, partly 
because I have already taxed your indulgence severely, and partly 
because the subject itself is so thorny a one, I mean the nature of 
variation. 
The facts that have been brought to light within recent years 
seem to point to the conclusion that two quite separate processes 
have lurked under the common denomination of variation. There 
is the variation that can be induced in any given species by the 
modification of the environment. Examples will occur to everyone. 
Thus stature, spine-development, even leaf form, may vary immensely 
within the limits of a species and the variation may frequently be 
correlated with special conditions, e.g. moisture, light, or tem¬ 
perature. It would appear in these cases that one is dealing with 
a constant specific mechanism that is able to be actuated indifferent 
ways by different kinds of stimuli. But there is another kind of 
variation now familiar under the name of mutation in which the 
matter is apparently different. A mutant breeds true under 
conditions which ought to reduce it to the ancestral type if it 
belonged to the catagory of the variations I have just mentioned. 
But this does not appear to be the case. Moreover, one may find a 
species that is actually, so to speak, throwing off mutations, as for 
example the mutation type called Qinothera lata , which De Vries 
has repeatedly obtained from the common Qinothera Lamarckiana. 
The difference between such a form and the parental type seems to 
lie in a change of constitution which is only another way of implying 
a substantive change in the chemical mechanism.—And with a 
change in the mechanism, the form , that is the expression of a 
modified material constitution, cannot resemble in every respect 
that of the unaltered type from which it sprang. 
Thus we might look on the first class of variations as the out¬ 
come of the operation of different stimuli on identical constitutions, 
whereas mutations would be recognised as the expression of the 
operation of identical stimuli on divergent constitutions. The 
strongest confirmation for such a view of the matter is perhaps that 
exhibited in the mosaic of hybrids. When the different sets of 
