chap, viil] MAMM.£ : COMMENT. 193 



sense, includes the cases of irregularity. The difficulty is to under- 

 stand the causes of regularity and of symmetry ; but if we could be 

 sure of these it would not be hard to conceive disturbances result- 

 ing in irregularity. 



In the pigs are found, first, cases of six on both sides in pairs, 

 and also of seven on both sides in pairs ; besides these there were 

 cases of 6 — 7 and of 7 — 8. Of these there were some in which two 

 on one side stood in positions which geometrically balanced that 

 of one of the other side, the others being arranged in pairs. In 

 such cases the appearances suggest that there has been a division 

 of one mamma to form two, and that the two have then separated 

 or travelled apart. The division of organs into two is of course a 

 common occurrence, and may naturally be supposed to be a pheno- 

 menon of the same nature as the division of single cells. The case 

 of mamma? is perhaps instructive inasmuch as it bears witness to 

 the fact that such division must take place at a remotely early 

 period in development. For while in cases to be given hereafter 

 of division, for example, between teeth, it may be supposed that 

 the travelling apart of the two resulting teeth is mechanical, in the 

 sense that the two growing teeth may simply push apart from each 

 other just as two cartilage- cells, &c, may separate by the concen- 

 tric deposition of material, the separation cannot be supposed to 

 occur in the mamma? by these late changes, but the process of 

 mechanical separation, though the same in kind as that in the case 

 of teeth, must be conceived as beginning early in the history of 

 segmentation. 



At this point a circumstance, very often to be seen in other 

 cases, should be mentioned. When an organ, single on one side, 

 corresponds geometrically with two organs on the other side, each 

 of the latter is frequently of the same size and developed to a like 

 extent as the single one of the other side. This of course would be 

 expected on the hypothesis that the division of organs is a pheno- 

 menon similar to the division of cells, that is to say, not merely 

 a division, but a reproduction. 



But the supposition of division of single members of the series 

 is not sufficient to account for all the facts of Variation seen. We 

 have to consider not only the case in which one organ of one side 

 balances two of the other. We have to deal also with the cases of 

 six on each side and seven on each side all corresponding in pairs. 

 In these there is no indication that there has been a division of a 

 single member on each side. The spacing is regular in each case 

 and there is no obvious crowding at any part of the series. Even 

 if therefore in the former case there is a suggestion that the genus 

 of single mamma? have divided into two at a period of develop- 

 ment after the series of mamma? was constituted as a series, there 

 is no such suggestion in the present case. We must, I think, in 

 the latter suppose that the existences of all the mamma?, whether 



b. 13 



