186 SUMMAEY 



of very old structures into something quite new; by 

 the frequent occurrence of useless vestigial struc- 

 tures; by the existence of annectent and synthetic 

 types ; and by the possibilities of "natural" classifica- 

 tion. 



8. In a general way the development of the indi- 

 vidual reads like a condensed recapitulation of the 

 racial evolution. In a general way the animal climbs 

 up its own genealogical tree. Gill-slits in the higher 

 Vertebrates are never used for respiration, and most 

 of them are of no use at all ; but they are never absent ; 

 they are historical relics. The persistence of the noto- 

 chord in the development of Amphibians, Reptiles, 

 Birds, and Mammals is another instance of embryonic 

 recapitulation, for this primitive skeletal axis is of 

 no permanent use for support above the level of 

 Fishes. In most cases it is very transient and is repre- 

 sented in the adult only by traces. The development 

 of important organs like brain, heart, and kidney 

 follows lines of historical evolution. Often there is a 

 remarkable circuitousness. The life-history of the 

 bony flat-fishes illustrates admirably the interpreta- 

 tive value of the evolution idea. 



9. That the past lives on in the present is an 

 evolutionist conclusion with abundant corroboration. 

 It is well illustrated in some of the habits of domesti- 

 cated animals, which are revivals of the ways of wild 

 ancestors. The story of the Common Eel is taken as 

 a vivid instance of the illumination of the present by 

 the past, for it becomes intelligible on the hypothesis 

 that this fish was originally at home in the deep sea 



