5 1 o Studies in the Theory of Descent. 



stage than in the other we also find more widely 

 divergent conditions of life ; wherever the mor- 

 phological systemy of one stage fails to coincide 

 with that of the other whether in the extent 

 or in the value of the groups the conditions of 

 life in that stage also diverge, either more widely 

 or at the same time within other limits ; whenever 

 a morphological group can be constructed from 

 one stage but not from the other, we find that this 

 stage alone is submitted to certain common con- 

 ditions of life which fail in the other stage. 



The law that the divergence in form always 

 corresponds exactly with the divergence in the 

 conditions of life 2 has accordingly received con- 

 firmation in all cases where we have been able to 

 pronounce judgment. Unequal form-divergences 

 correspond precisely with unequal divergence in 

 the conditions of life, and community of form 

 appears within exactly the same limits as com- 

 munity in the conditions of life. 



These investigations may thus be concluded 

 with the following law : In types of similar origin, 

 i.e. having the same blood-relationship, the degree 

 of morphological relationship corresponds exactly 



* [Eng. ed. This law is perhaps a little too restricted, inas- 

 much as it is theoretically conceivable that the organism may 

 be able to adapt itself to similar conditions of life in dif- 

 ferent ways ; differences of form could thus depend sometimes 

 upon differences of adaptation and not upon differences in the 

 conditions of life, or, as I have formerly expressed it, it is not 

 necessary to allow always only one best mode of adaptation.] 



