192 MERISTIC VARIATION. [part i. 



general segmentation of the trunk. The same author then argues 

 that the appearance of supernumerary nipples or mamma? along 

 the mammary lines is a reversion to an ancestral condition, and a 

 figure is given, shewing the places at which mamma? are on this 

 view believed to have been placed, definite ordinal numbers being 

 assigned to each. Against this suggestion may be urged those ob- 

 jections to appeals to the hypothesis of reversion which were men- 

 tioned in the Introduction (Section XII.), but in addition to these 

 there are a number of objections applying specially in the case of 

 mammary Variation. The view that supernumerary mamma? are 

 reversions rests on the frequency and definiteness with which they 

 occupy certain positions. But though they do occur more often 

 in some positions than in others they are in no sense limited to 

 these positions, for they may stand anywhere, at least upon the 

 mammary lines. To justify the view that the positions of super- 

 numerary mamma? are definite it is necessary to exclude the cases 

 of bifid nipple, of multiple nipples on the same breast, and of axil- 

 lary extensions of the mamma?, all which phenomena would then 

 be looked on as belonging to a class different from that of actual 

 supernumerary mamma?. In the argument referred to, this course 

 is actually adopted. The acceptance of such a view leads to great 

 difficulty. For example, in Neugebauer's case (see Fig. 29), 

 Williams considers that the posterior nipples of the two sides be- 

 long to different pairs, and have consequently different homologies, 

 because they stand at different levels. 



Such distinctions are, I believe, unreal. It is surely impossible 

 to suppose that the Repetition seen in the udders of the two cows 

 in Fig. 32 is a phenomenon different in the two cases. In the one 

 there are two extra teats in symmetrical positions, equally spaced 

 out from the second teats ; in the other there is a third teat on 

 one side and a double second or posterior teat on the other. Surely 

 it is clear that the double condition of this teat represents an im- 

 perfect phase of a process perfected on the other side. If further 

 proof were needed it may be found in the fact already mentioned, 

 that the mamma? of the pig and other such animals, may be the 

 same in number even on the two sides, but nevertheless stand 

 quite irregularly and without any visible arrangement into pairs. 



The existence of these cases in which no order of form or regul- 

 arity can be traced may seem at first sight to be an insuperable 

 objection to any attempt at the detection of principles in the ar- 

 rangement of the mamma?. There is however the fact that many, 

 and indeed in most forms the majority of individuals do shew an 

 orderly and paired arrangement, and the further fact that of those 

 cases which depart from this, a certain number present appear- 

 ances which suggest that this departure has come about in a regular 

 way. Though the irregular cases remain, something would be 

 gained if we could comprehend any of the elements on which the 

 regularity depends. The case of regularity and symmetry, in a 



