290 THE RECAPITULATION THEORY 



L_Embryology, referred to by the greatest of 

 naturalists as " one of the most important subjects 

 in the whole round of Natural History," is still in 

 its youth, but has of late years thriven so mightily 

 that fear has been expressed lest it should absorb 

 unduly tfie attention of zoologists, or even check 

 the progress of science by diverting interest from 

 otheFjmjDsqually important branches. Nor is the 

 reason of this phenomenal success hard to find. 

 The actual study of the processes of development ; 

 the gradual building up of the embryo, and then of 

 the young animal, within the egg ; the fashioning 

 of its various parts and organs ; the devices for 

 supplying it with food, and for ensuring that the 

 respiratory and other interchanges are duly per- 

 formed at all stages : all these are matters of 

 absorbing interest. Add to these the extraordinary 

 changes which may take place after leaving the 

 egg, the conversion, for instance, of the aquatic 

 gill-breathing tadpole a true fish as regards all 

 essential points of its anatomy into a four-legged 

 frog, devoid of tail, and breathing by lungs ; or 

 the history of the metamorphosis by which the 

 sea-urchin is gradually built up within the body of 

 its pelagic larva, or the butterfly derived from its 

 grub. Add to these again the far wider interest 

 aroused by comparing the life-histories of allied 

 animals, or by tracing the mode of development of 

 a complicated organ e.g., the eye or the brain in 

 the various animal groups, from its simplest com- 

 mencement, through gradually increasing grades of 

 efficiency, up to its most perfect form as seen in 



