RECAPITULATION 55 



ably man has descended from a water-breathing animal ; yet 

 his embryo never very closely resembles one. It has changed 

 from its prototypes of the life-history. Incapable of a 

 separate existence, it is adapted to live only as a parasite in 

 its own special environment. Its prototypes, on the other 

 hand, could not have existed thus. Immense changes have 

 taken place. The explanation lies in the fact that variations 

 occur, not only at the end of development, but during the course 

 of it. The child differs from the parent as a germ, an embryo, 

 and a foetus, as well as during adult life. During every 

 stage evolution is at work, preserving useful variations and 

 eliminating harmful structures. When the environment 

 changes for the adults of the species it changes for the em- 

 bryos also. Step by step the race in all its developmental 

 stages follows the change in the environment. To-day, for 

 example, mammals retain their young within their bodies for 

 prolonged periods. The whole structure of the embryo has 

 altered in consequence. Thus the gills have almost disap- 

 peared and a placenta has undertaken their function. Among 

 the medusae there is alternation of generations. The egg gives 

 rise to a larva, the larva to a polype, the polype to more 

 polypes by budding, each of these to many jelly-fish, the jelly- 

 fish to many eggs. In some species one or other stage is now 

 omitted. " In the life-history of the Hydroidae any phase, 

 planuloid, polypoid, or medusoid, may be absent." 1 Thus the 

 nauplius stage has been suppressed among nearly all mala- 

 costracan crustaceans. Doubtless these changes did not 

 occur in monstrous forms ; for example, a larva whose ancestors 

 had given rise to polypes did not suddenly omit the polype 

 stage and give origin to a jelly-fish. On the contrary, all our 

 knowledge of Nature leads us to believe that the alteration 

 occurred gradually during many generations. The polype 

 stage was slowly shortened till at length it disappeared. In 

 this instance whole chapters, or volumes rather, have been 

 omitted from the life-history. If a plant, for instance a straw- 

 berry, propagated unendingly by means of runners only, all the 

 earlier volumes of the life-history of its race would be lost. In 

 fact budding may be regarded generally as a device to get rid 

 of unnecessary chapters to avoid unnecessary recapitulation. 

 92. We see then that there is recapitulation, but not exact 

 or complete recapitulation. The development of every indi- 

 vidual, except for his own variations, is a complete recapitula- 

 tion of the parent's development, but it is not a complete 



1 Dr. Strethill Wright, quoted by Darwin, Animals and Plants, 

 vol. ii., p. 364. 



