58 THE THEORY OF EVOLUTION 



It was very early recognized that the recapitula- 

 tion theory could not be applied with literal exact- 

 ness, but was subject to certain important excep- 

 tions and qualifications. 



(1) That the history must have been enormously 

 abbreviated. After three weeks of incubation the 

 tiny speck of protoplasm, which forms a circular 

 mark on the yolk of a hen's egg, is developed into a 

 fully formed chick, ready for hatching and able in 

 large degree to take care of itself. On the other 

 hand, the evolution of birds from their invertebrate 

 ancestors, through the fishes, amphibians, and rep- 

 tiles, the separation of the gallinaceous stock from 

 other birds and the differentiation of this particular 

 species were extremely slow processes, extending 

 through unnumbered millions of years. Admitting 

 recapitulation to the fullest extent, it is evidently 

 a physical impossibility that it should be a perfect 

 repetition of phylogeny; very much of the long story 

 must of necessity be omitted. 



(2) Through all the stages of development the 

 embryo must be rendered able to live and grow and 

 thrive through adaptation to its surroundings and 

 changes in its environment. In some animals de- 

 velopment takes place within the body of the mother; 

 in others the embryo is protected by the hard egg- 

 shell, as in birds, while the eggs of certain fishes and 

 many invertebrates float freely in the sea and are 

 almost without protection. Such differences in en- 

 vironment necessitate differences in the mode of 



