. ORGANIC EVOLUTION 47 



wing-bars, like the wild rock-dove, from which all 

 domestic breeds have sprung. In man the cheek- 

 bone and the frontal bone of the forehead consist 

 normally each of a single bone. But in children 

 and human embryos these bones are always double, 

 as is normally the case in adults among some of 

 the anthropoids and other mammals. Gills appear 

 regularly in the embryos of reptiles, birds, and 

 mammals, and human young are sometimes born I 

 with gill-slits on the neck. There are times when, ' 

 owing to inaccurate or incomplete embryological 

 development, these fish-like characteristics are so 

 perfect at birth as to allow liquids, on being 

 swallowed, to pass out through them and trickle 

 down on the outside of the neck. Many muscles 

 are occasionally developed in man which are 

 normal in the apes and other mammals. As 

 many as seven different muscular variations have 

 been found in a single human being, every one of 

 which were muscles found normally in the struc- 

 ture of the apes (8). 



5. Closely akin to atavism, which is the occa- 

 sional persistence of ancestral types of character, 

 is the regular occurrence of vestigial organs or 

 structures, organs which in ancestral forms have 

 definite functions, but which in existing species, 

 owing to changed conditions, are rudimentary and 

 useless. On the back of each ankle of the horse 

 are two splints, the atrophied remains of the second 

 and fourth toes. Similar vestiges of two obsolete 

 toes are also found just back of the wrists and 

 ankles on all the two-toed ungulates, such as the 



