DARWIN ON THE ORIGIN OF SPECIES 127 



create classifications which admit of being- thrown into 

 the form of genealogical trees. Wide gaps in the 

 geological history of life render the task difficult beyond 

 expression, but much has already been accomplished. 

 Newly discovered forms (especially the fossil Archaeop- 

 teryx and the Cycadofilices) and more fully investigated 

 forms, far too numerous to be specified, have estab* 

 lished links between groups which formerly seemed to 

 be wholly independent. Unnatural assemblages based 

 on pre-determined characters (Radiates, Entozoa, Birds 

 of prey, etc.) have been replaced by groups which are 

 at least possible on evolutionary principles. Almost 

 every working naturalist will admit that the progress 

 of zoological and botanical system during the last two 

 generations has done much to fortify the Darwinian 

 position. 



2. Embryology. Baer in 1828 was possessed of all 

 the embryological facts which Darwin used in support 

 of his theory of evolution ; in particular, he was well 

 acquainted with the most striking fact of all viz., the 

 presence in embryo mammals and birds of a series of 

 paired clefts along the sides of the neck, between which 

 run vessels arranged as in gill-breathing vertebrates* 

 The vessels had been figured by Malpighi ; the clefts 

 had been discovered by Rathke, who had no hesitation 

 in calling them gill-clefts and the vessels gill-arches. 

 Nor had Baer, who nevertheless to the end of his long 

 life refused to accept the one explanation which gives 

 meaning to the facts viz., that remote progenitors of 

 mammals and birds breathed by gills. Few embryo- 

 legists have since felt such a scruple. The adaptation 

 to gill-breathing is obvious ; is gill-breathing now 

 practised by any mammal or bird? Certainly not. Is 

 it destined to be practised by their descendants at 



