204 Experime7ital Zoology 



Elimination within the Species 



Although elaborate data have been published in respect to 

 fluctuating variation and ehmination 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 ehmina- 

 tion in each, hence in many cases nothing appears to be gained 

 permanently by the ehmination 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 

 hght on the formation of new species, they will be of much 

 importance as a contribution to the study of variation within 

 the hmits 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 maenas, 

 found hving in the harbor of Plymouth. The measurements 

 were made in the years 1893, 1895, 1898, and show, during this 

 time, a decrease, which he beheves to be due to ehmination 

 brought about by a change in the water of the sound. The mean 

 of the frontal breadth for 1895 was less than for 1893, ^^^ a 

 further change was found in 1898. These resuhs are for male 

 crabs. The changes in the female were less in amount. The 

 ehmination is due, Weldon thinks, to the fine mud, brought 



