IX THE HISTORY AND SCOPE OF ZOOLOGY 349 



existing forms from lower to higher, and (c) the series 

 of forms which succeed one another in the strata of 

 the earth's crust; whilst an explanation of this parallel- 

 ism was either not attempted, or was illusively offered 

 in the shape of a doctrine of harmony of plan in crea- 

 tion. It was the application of Fritz Miiller's law of 

 recapitulation which gave the chief stimulus to recent 

 embryological investigations ; and, though it is now 

 recognised that " recapitulation " is vastly and be- 

 wilderingly modified by special adaptations in every 

 case, yet the principle has served, and still serves, as 

 a guide of great value. 



Another important factor in the present condition 

 of zoological knowledge as represented by classification 

 is the doctrine of degeneration propounded by Anton 

 Dohrn. Lamarck believed in a single progressive 

 series of forms, whilst Cuvier introduced the conception 

 of branches. The first post-Darwinian systematists 

 naturally and without reflection accepted the idea 

 that existing simpler forms represent stages in the 

 gradual progress of development, are in fact survivors 

 from past ages which have retained the exact grade 

 of development which their ancestors had reached in 

 past ages. The assumption made was that (with the 

 rare exception of parasites) all the change of structure 

 through which the successive generations of animals 

 have passed has been one of progressive elaboration. 

 It is Dohrn's merit to have pointed out 1 that this 

 assumption is not warranted, and that degeneration or 



1 Ursprung der Wirbelthiere, Leipsic, 1875 ; and Lankester, De- 

 generation, London, 1880 (reprinted in the present volume). 



