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CHAPTER III. 



SIMILARITY OF SUCCESSIVE TYPES. 



ONE of the strongest arguments in favour 

 of the Darwinian theory is based on the 

 similarity in structure of successive types. 



In many cases the skeleton and general 

 organisation of the antecessor and of the suc- 

 cessor are all but identical, and in every case 

 the embryo of a successor presents, during its 

 growth, phases similar to what may be ob- 

 served in that of its antecessor. Indeed, in 

 some instances some breeds of dogs, for 

 example the young of the successor and of 

 the antecessor cannot at birth be distinguished 

 from each other, and the specific character- 

 istics of the successor are only developed 

 between birth and maturity. 



Seeing that the differentiation, between 

 an antecessor and a successor, begins invari- 

 ably at the same stage in the growth of their 

 embryos, may we not naturally infer that the 

 differentiation is due to new forces in the 







