THE RECAPITULATION THEORY 305 



important groups, the true affinities of which were 

 first revealed by embryology ; and in the case of 

 parasitic animals the structural modifications of the 

 adult are often so great that, but for the evidence 

 yielded by development, their zoological position 

 could not be determined. It is now indeed gene- 

 rally recognised that in doubtful cases embryology 

 affords the safest of all clues, and that the zoo- 

 logical position of such forms can hardly be 

 regarded as definitely established unless their 

 development, as well as their adult anatomy, is 

 ascertained. 



It is owing to this Recapitulation Theory that 

 Embryology has exercised so marked an influence 

 on zoological speculation. Thus the formation in 

 most, if not in all, animals of the nervous system 

 and of the sense organs from the epidermal layer 

 of the skin, acquired a new significance when it 

 was recognised that this mode of development was 

 to be regarded as a repetition of the primitive 

 mode of formation of such organs ; while the 

 vertebral theory of the skull affords a good ex- 

 ample of a view, once stoutly maintained, which 

 received its death-blow through the failure of 

 embryology to supply the evidence requisite in 

 its behalf. The necessary limits of time and space 

 forbid that I should attempt to refer to even the 

 more important of the numerous recent discoveries 

 in embryology, but mention may be very pro- 

 perly made here of Sedgwick's determination of 

 the mode of development of the body cavity in 

 Peripatus, a discovery which has thrown most 



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