476 The American Naturalist. [June, 



view is negatived by the fact that this muscle is innervated by 

 the anterior thoracic (Cunningham, Shepherd) which would 

 connect it with the pectoral system, or by the intercostal 

 nerves (Bardeleben). Although the high percentage of recur- 

 rence in the sternalis in anencephalous monsters (ninety per 

 cent, according to Shepherd) support- the reversion view, it is 

 offset by the high percentage (four per cent.) in normal sub- 

 jects, 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 variations, as well as variations in the pro- 

 portions of organs, play an important part in the present 

 discussion upon heredity, for it is believed by the Weismann 

 school that such variations, if they chance to be useful, will be 

 accumulated by selection and thus become race characters. 



The Limits of Reversion. — There is such a wide difference 

 of opinion upon the 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 sternalis muscle, which strain the 

 reversion theory to the breaking-point ? 



Testut, 1 Duval, and Blanchard take the extreme position 

 that almost all anomalies reproduce earlier normal structures, 

 and that the exceptions may be attributed to the incomplete- 

 ness of our knowledge of comparative anatomy. I may here 

 observe that popular as the descent theory has recently 

 become in France, neither these anthropologists nor the 

 palaeontologists show a very clear conception of the phyletic 

 or branching elements in evolution. If they do not find a 

 muscle in the primates they look for it in other orders of 

 mammals. Now, since these other branches diverged from 

 that which gave rise to man at a most remote period, the dis- 



l Op. cit., p. 4. 



