2 2 BIOLOGICAL LECTURES. 



phenomena, to sift out adaptations to existing conditions from 

 those which can only be comprehended by reference to former 

 conditions. Phenomena of the latter class may, for the sake of 

 brevity, conveniently be termed "ancestral reminiscences," — 

 though it may not be superfluous to remark that every char- 

 acteristic of the organism is in a broad sense reminiscent qf 

 the past. 



It is in embryological development that ancestral reminis- 

 cence is most familiar and most striking. We all know that 

 development rarely takes the shortest and most direct path, 

 but makes various detours and sometimes even moves backward 

 so that the adult may actually be simpler than the embryo. 

 Such vagaries of development are in many cases only intelli- 

 gible when regarded as reminiscences of bygone conditions, 

 either of the adult or of the embryo. Sometimes these records 

 of the past are so consecutive and complete that the individual 

 development, or ontogeny, may be said to repeat or recapitulate 

 the ancestral development, or phylogeny. The development of 

 the toad's ^gg, for example, probably gives in its main outlines 

 a fairly true picture of the ancestral history of the toad race, 

 which arose from fish-like ancestors, developed into aquatic air- 

 breathing tailed forms, and finally in their last estate became 

 tailless terrestrial forms. It was such facts as these that led 

 Haeckel, building on the basis laid by Darwin and Fritz Miiller, 

 to the enunciation of the famous so-called "biogenetic" law, 

 that the ontogeny, or history, of the individual tends to repeat in 

 an abbreviated and more or less modified form the phylogeny, or 

 history, of the race. The event has shown that actual recapitu- 

 lation or repetition of this kind is of relatively rare occurrence. 

 Development more often shows, not a definite record of the 

 ancestral history, but a more or less vague and disconnected 

 series of reminiscences, and these may relate either to the adult 

 or to the embryonic stages of the ancestral type. Thus the 

 embryo mammal shows in its gill-slits and aortic arches what 

 must probably be regarded as reminiscences of a fish-like adult 

 ancestor, while in the primitive streak it gives a reminiscence 

 not of an adult form but of an ancestral mode of development 

 from a heavily yolk-laden Qgg like that of the reptiles. 



