Succession of Ontogenetic Stages 15 



In other words the biogenetic. law implies that up to 

 each stage of development the productive cause of develop- 

 ment remains the same as that which produced the 

 ancestral species corresponding to that stage. 



We should next ascertain whether the new circum- 

 stance now added or the new force becoming active only 

 at this stage and causing the subsequent development is 

 to be sought for within or without the various parts of 

 the organism which are actually in process of formation. 



If at the start we limit ourselves for the sake of ' 

 simplicity to the consideration of morphological trans- 

 formations only, each stage of development whether 

 ontogenetic or phylogenetic will appear to us only as a 

 special mode of distribution of the organic substance con- 

 stituting the organism. \ But this distribution is modified 

 during the life of the adult individual only by new func- 

 tional stimuli, that is to say, only by agents which are 

 external to the structure in progress of modification. In 

 other words the impulse by which the corresponding por- 

 tion of living organic substance is constrained to 

 distribute itself differently does not reside within this 

 portion but comes to it from without. 



Until the contrary is proven we may accept the state- 

 ment that the properties of living organic substance dur- 

 ing development are not different essentially from those 

 which it presents when development is completed. Con- 

 sequently when any particular mode of distribution of 

 the organic substance becomes altered during the prog- 

 ress from one given ontogenetic stage to the succeeding 

 stage, we can admit as a provisional hypothesis that this 

 different distribution is effected by some provocation 

 external to the parts which change. 



This provocation cannot be constituted merely by the 



