328 PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 



inot(>. The same distinction aiiplies to polydactylisin. How absurd 

 it is to consider a sixth lin,i;-er atavistic, when we remember that even 

 our Permian ancestors had but five lingers. 



We can not however class, as purely fortuitous a variation which 

 occurs in a definite percentage of cases presenting twenty four differ- 

 ent varieties, but oc<'arring in the same region. Such is the much-dis- 

 cussed* musculus sternalis, a muscle extending vertically over the 

 origin of the pectoralis from the region of the sterno-mastoid to that 

 of the obliquus externus. Testut lightly applies liis universal rever- 

 sion theory, and as this uniscle is not found in any mammal considers 

 it a regression to the rei)tilian presternal (0]»hidia) ! Turner also con- 

 sidered it as reversional in connection with the panniculus carnosus, 

 the old twitching muscle of the skin, which plays so many freaks of 

 reversion in the scalp and neck; this view is negatived by the fact that 

 this muscle is innervated by the anterior thoracic (Cunningham, Shep- 

 herd) which would c(mnect it with the pectorial system, or by the inter- 

 costal nerves (Bardelebeu). Although the high percentage of recur- 

 rence in the sternalis in anencei)halous monsters (90 percent according 

 to Shepherd) sui)ports the reversion view, it is offset by the high per- 

 centage (4 per cent.) in normal subjects, for this is far too high for a 

 structure of such age as the reptilian presternal. Cunningham has 

 advanced another hypothesis, first suggested by the frequency of this 

 anomaly in women, that this is a new inspiratory muscle, having its 

 origin in reversion, but serving a useful purpose when it recurs, and 

 therefore likely to be perpetuated. 



These fortuitous \ariations, as well as variations in the proi)ortions 

 of organs, play an important part in the present discussion upon hered- 

 ity, for it is believed by the Weismann school that such variations, if 

 they chance to be useful, will be accunndated by selection and thus 

 become race characteis. 



The limits of rrverfiion. — There is such a wide difference of opinion 

 upon tlie subject of reversions that it is important to determine what 

 are some of the tests of genuine reversions. How shall we distinguish 

 them from indefinite variations or from anomalies like the steriuilis 

 muscle, which strain the reversion theory to the breaking i)oint? 



Testut, f Duval, and Blanchard take the extreme position that almost 

 all anomalies reproduce earlier normal structures, and that the excep- 

 tions may be attributed to the incompleteness of our knowledge of 

 coini>arative anatomy. I may here observe that ])opular as the descent 

 theory has recently become in France, neither these anthroi)ologists 

 nor the paliTeontologists show a very clear conception of the phyletic 

 or branching element in evolution, if they do not find a nniscleinthe 

 primates they h)ok for it in other orders of mannnals. Now, since 

 these other branches diverged from that which gave rise to man at a 



*See Turner, Shepherd, niid C'nnniughnm: Jourvot of Aiinfomti and rhjisinlo;!;/. 

 i Siir lea Anomalies MiisciilaircK, p. 4. 



