204 Experimental Zoology 
Elimination within the Species 
Although elaborate data have been published in respect to 
fluctuating variation and elimination within a given generation, 
comparatively few data have been gathered to show what occurs 
in successive generations under identical, as well as under differ- 
ent, conditions. Haphazard mating seems to produce the same 
variation in successive generations, despite the accidental elimina- 
tion in each, hence in many cases nothing appears to be gained 
permanently by the elimination of the unsuccessful or of the 
unfortunate individuals. In order to discover to what extent 
the causes of fluctuating variation are internal and to what extent 
external, we need experiments carried out through several gen- 
erations on pedigree animals, selectively paired, and kept under 
constant conditions. While such results may or may not throw 
light on the formation of new species, they will be of much 
importance as a contribution to the study of variation within 
the limits of the species. 
The elimination of individuals within each generation may be 
due to catastrophes that overwhelm all the individuals involved, 
without regard to their individual differences, and the great 
destruction of immature individuals appears to belong largely to 
this class; or the elimination may follow the lines of individual 
differences which gives a selective value to the survivors. Which 
of these, or of other kinds of elimination, occur even for those 
forms that have been studied statistically, is known only incom- 
pletely in a few cases. 
Weldon has measured the frontal breadth of Carcinus menas, 
found living in the harbor of Plymouth. The measurements 
were made in the years 1893, 1895, 1898, and show, during this 
time, a decrease, which he believes to be due to elimination 
brought about by a change in the water of the sound. The mean 
of the frontal breadth for 1895 was less than for 1893, and a 
further change was found in 1898. These results are for male 
crabs. The changes in the female were less in amount. The 
elimination is due, Weldon thinks, to the fine mud, brought 
