84 THE DARWINIAN THEORY 



essential features. Embryology tells us that this 

 means that the three animals must really be members 

 of the same group, and allied to one another; and 

 thereby gives us a clue to the real affinities of bar- 

 nacles and Sacculina that we could hardly get in any 

 other way. (Figs. 9, 10, 11.) 



In parasitic animals generally the shape and struc- 

 ture are liable to be so profoundly modified, in con- 

 sequence of -the special conditions of parasitic exist- 

 ence, that, but for the aid afforded by development, 

 we should often be absolutely unable to determine 

 to which division of the animal kingdom they really 

 belong. 



This leads us to our second question : If Embryo- 

 logy gives the clue to the relationship of a nimals, 

 may it not do more and reveal their ancestry ? 



The Recapitulation Theory. 



A further explanation is afforded us by what is 

 known as the Recapitulation Theory, which states that 

 not merely have existing animals descended from 

 ancestors which are often unlike them, but that 

 each animal bears the mark of its own ancestry and 

 reveals its parentage in its own development. Evo- 

 lution tells us that each animal has had a pedigree 

 in the past ; Embryology^ reveals to us th is an cestry, 

 because every animal has an inher ited^ tendency 

 during its own development to_repeat i ts ow n_ ances- 

 ytraMiistory ; or, to put it in other words, to climb 

 up its own genealogical tree. 



A good example of recapitulation is afforded by 



