86 Walter Stapley : 



He is iilso in eri-or when lie says: — "In many i)ii-ds and reptiles 

 all the cervical vertebrae bear ribs," whereas many birds, ])articularly 

 ostriches, show large vestigial neck ribs. Owen in lS6f) had correctly 

 written: — ^" In the cervical vertebrae of birds the pletirapophysis, if 

 present, is confluent with the neural arch." 



Gegenbaur refused to accept the old ideas of atavism and laid it 

 down that: — "Atavistic parts do not belong to forms palaeontcjlo- 

 gically or systematically far distant." 



Sutton, who accepted Gegenbaur's restrictions, grouped atavistic 

 phenomena into two classes:— (1) "The attainment of functional con- 

 dition by structure normally suppressed." (2) " Reversion of organs 

 and tissues to an original type." 



Although Darwin had no difficulty in understanding the extinction 

 of species, he realised that he was unable to understand the extinction 

 of organs and structures. The following quotations from Darwin's 

 writings bear out this point: — "It is most difficult always to re- 

 member that the increase of every creature is constantly being checked 

 by unperceived hostile agencies, and that these agencies are amply 

 sufhcient to cause rarity and finally extinction." 



" There remains, however, this difficulty after an organ ha,s ceased 

 being used, and becomes in consequence much reduced, how can it be 

 still further reduced in size until the merest vestige is left ; and how 

 can it be finally quite obliterated? It is scarcely possible that disuse 

 can go on producing any further effect after the organ has once been 

 rendered functionless. Siune additional exjvlanation is here requisite 

 which I cannot give." 



The theory of atavism does not fit the evolution of cervical ribs ; 

 the old idea takes us back to the early lizards and fish, the modern 

 idea to the ijuadrupedal mammals, and the new idea is distinctly 

 Mendelism. 



The destruction nf neck ribs can lie traced from the tish thmugh 

 the reptiles and monotremcs until in the mammals thev become small 

 and confiuent with the neck bones. From the quadrupedal mammals 

 through some of those mammals that have abandoned (iuadru]>edal 

 gait,. notal)ly man and sirenia, cervical rib develojmient may he traced 

 from small undeveloped cervical ribs that appear only occasionally 

 in the dugfing, to fully formed cervical ribs that occur constantly in 

 (he manatee. 



,Thc point of greatest interest in relation tn maanmaliau cervical 

 rib.s is the seventh cervical vertebra ; a careful study of this bone 

 shows that ribs undergo not oidy sujiprcssion, but that they also 

 undergo extinction, for when the seventh cervical vertebra is without 

 costal jM-ocesses, il is evident tliat the rilt vestiges have disappeared 

 and that the ribs, which in an earlier stage of evolution belonged to 

 this bone, have become extinct. 



