Succession of Ontogenetic Stages I5 
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 
