IO2 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 



