W. E. Agar 187 



(i.e. in their first instar), for the last 44 generations, the firat two not 

 being available for this pui-pose. Only first broods are dealt with in the 

 table, and those portions of the line living under the abnormal conditions 

 described above are omitted, as these conditions (h'l-ectly affected the 

 size of the new-born young. 



It will be seen that the size fluctuates greatly, but there is no real 

 tendency to a progressive . increase. If the forty-four generations are 

 divided into four groups of eleven each, we find : 



Mean length for generations 3 — 13 43"637 



14—24 43-017 



25—35 44-423 



36—46 44-457 



It will be seen that the second set of eleven generations average 

 smaller than the first. It is true that the last period gives the highest 

 average, thanks to generation 36 which was represented by only a single 

 brood of exceptionally large individuals. The mean of the last six 

 generations however is 43-936, almost exactly equal to the total mean 

 (43-889) and so is the mean for the first six, 44-104. There is thus 

 no justification for supposing that the size of the young is increasing, 

 although by taking twenty successive generations (Papanicolau's number) 

 a good case might be made out for either increasing or decreasing size 

 according to the portion of the table included by them. 



I can fully confirm Papanicolau's statement that the size of the egg 

 (or new-bom young) increases from brood to brood — at any rate up to 

 about the fifth. Exact measurements of this increase were given in my 

 former paper. This increase however is in no sense due to a cyclical or 

 cumulative action of the increasing lapse of time since the last sexual 

 act. It is a phenomenon which begins afresh in each generation, and 

 seems to be a common occurrence throughout the animal kingdom. The 

 increasing size of the eggs of successive ovipositions has been shown in 

 detail by Halban for Salamandra maculosa and S. air a, and also for 

 Anura and Chelonia. It is the experience of every poultry keeper that 

 the young hens lay smaller eggs than the older ones, and it is well 

 known to be the case with man that the size of the new-bom infant 

 tends to increase in successive parturitions. 



Whilst the line described above was in progress I also bred numerous 

 others, the most important being two started respectively from a par- 

 thenogenetic female kindly sent me from Cambridge by Mr F. A. Potts 

 and from one from Beith, 16 miles from Glasgow, kindly sent to me by 



