1892.] The Contemporary Evolution of Man. All 



covery of a similar muscle may be merely a coincidence ; it, is 

 by no means a proof of reversion. 



The first test of reversion is therefore the anatomy of the 

 atavus, and this is derived partly from the puhvontological 

 record of the primates, partly from the law of divergence, viz., 

 that features which are common to all the living primates 

 were probably also found in the stem form which gave rise 

 to man ; finally, from the comparative anatomy of the living 

 anthropoidea. 



The second test is whether a structure passes the limits «.f 

 reversion as determined by cases of atavism in which there 

 can be no reasonable doubt. Two of these phenomena have 

 recently been discussed, which seem to extend the possibilities 

 of reversion back to structures which were lost at a very 

 remote period. I refer to papers by Williams and Howes. 

 Williams 1 has analyzed 166 recorded cases of polymastism ; 

 he finds that supernumerary nipples of some form occur in 

 two per cent., and that in all except four of the cases exam- 

 ined the anomalies, tested by position, etc., support the rever- 

 sion hypothesis. In the living lemurs, which form a persist- 

 ent primitive group of monkeys, we find that the transition 

 from polymastism to bimastism is now in progress by tin- 

 degeneration of the abdominal and inguinal nipples, it is fair 

 to assume that the higher monkeys also lost their abdominal 

 nipples at a primitive stage of development, and therefore that 

 cases of multiple nipples indicate reversion to a lower Eocene 

 condition ! Howes 2 has recently completed a most interesting 

 study of the " intranasal epiglottis," or cases in which the epi- 

 glottis is carried up into the posterior nares, as m young mar- 

 supials and some cetacea, to subserve direct narial respiration. 

 This has now been observed to occur by reversion in all orders 

 of mammals, including the monkeys and lemurs. Or 

 has also been reported by Sutton of its c 

 foetus. This is apparently a human reversi 

 much older than the age of the lemurs. 



The third test is the inverse ratio to time. 

 priori, that the percentage of recurrence ot ; 



journal of Anatomy and Physiology, 1891, p. 224. 

 *Ibid, 1889, p. 587. 



i a hum a 



