102 FOOT-NOTES TO EVOLUTION. 
ment. Phenomena once supposed to be due entirely to 
intrinsic causes are now known to be the result of ex- 
trinsic ones, and it is practically certain that this will be 
found true of still other phenomena. But although it 
is not possible to draw any hard and fast line between 
these two classes of causes, one can, in general, recog- 
nise a very marked difference between them. Extrinsic 
causes may, in large part, supply the stimulus and the 
energy for development, and may more or less modify 
its course; the intrinsic causes are of a much more com- 
plex character than the extrinsic ones, they are inher- 
ent in the living matter and in large part predetermine 
the course of development. In one form or another the 
distinction between these two classes of causes is recog- 
nised by all naturalists. Professor His calls the intrinsic 
causes “ the law of growth,” the extrinsic ones the con- 
ditions under which that law operates. These designa- 
tions correspond, at least in part, to Professor Cope’s 
anagenesis and katagenesis, and to Roux’s “ simple and 
complex components” of developmental processes. 
While it is necessary to emphasize the differences 
between these two classes of causes, it is not intended 
thereby to dogmatically assert their total difference in 
kind. It may well be that these extrinsic and intrinsic 
causes are totally different in kind, but in our present 
state of ignorance it would be unjustifiable to affirm it. 
On the other hand, it would be just as unwarrantable to 
dogmatically affirm that there is no difference in kind 
between these two classes of causes, and that, therefore, 
all vital phenomena are only the manifestations of heat, 
light, electricity, attraction, repulsion, chemism, and the 
like. It may be that this is true, but there is as yet no 
sufficient evidence for it, and to attempt, as certain 
dynamical and mechanical hypotheses do, to refer all 
vital phenomena directly to such simple components as 
