48 A NEW THEORY OF EVOLUTION. 



These footprints of evolution seem to 

 present a clue to the method of its prog- 

 ress ; and to explain these phenomena 

 satisfactorily, is an important test of any 

 theory of evolution. 



According to our theory, the phases in 

 the growth of an embryo should resemble 

 the corresponding phases in the embryo of 

 its antecessor, up to the stage when the 

 Specific life-force of the successor begins to 

 differentiate the new type. When the 

 differentiation between the old type and 

 the new is slight, as in the case of what 

 are called varieties of the same species, the 

 modification of the pre-existing Specific life- 

 force is slight, and the embryos are con- 

 sequently similar up to an advanced stage 

 in their growth. 



In cases where the differentiation between 

 two types arises only after birth, the mature 

 fostus of the one is not distinguishable from 



o 



that of the other. But when the differen- 

 tiation is important and extends to several 

 organs, the co-operation of the old and the 

 new Specific life -forces becomes more com- 

 plicated. The sequence of the growth in the 

 embryo of the organs of the old type and of 

 the new is not always the same, and in such 

 cases two forces seem for a time to be in 



