228 HAECKEL 



mental norm, which is made clear to us in 

 embryology and can at the same time (remember 

 the instance of the lizard-like teeth in the bird- 

 embryo) give us most wonderful suggestions as to 

 the line of ancestral development. But it has 

 certain limitations, as we will now show. 



The adaptations in the sense of the Darwinian 

 laws have affected the animal's embryonic life more 

 and more, the higher the tree of life grew. The 

 long recapitulation of the ancestral stages often 

 came into conflict with the young individual's need 

 for protection. The result was that the biogenetic 

 law found itself restricted by the Darwinian laws 

 of adaptation. The too lengthy succession of an- 

 cestral portraits was abbreviated and compressed. 

 Whole stages of embryonic or larval development 

 were interpolated that had nothing to do with 

 these ancestral portraits, but were destined for the 

 protection of the foetus. The butterfly-pupa is 

 really an instructive instance of this description. 

 It does not reproduce a crab-stage, nor has there 

 been any stage in the ancestry of the butterfly 

 when they lived throughout life in pupa-houses. 

 The pupa is simply a later adaptation in the 

 development of the butterfly, a protective stage in 

 which it accomplishes the transition from the 

 caterpillar-form in much the same way as the 

 young bird develops under the protection of the 

 hard egg-shell. Thus only a faint and shadowy 

 trace has been left of the real ancestral forms, 

 though this trace is an extremely instructive one. 

 But we must not expect the impossible from it. 



