Influence of Selection 205 
down by the rivers that became greater in amouat during these 
years. He tested this view by keeping crabs in water contain- 
ing the same mud in suspension. The crabs that died were on 
the whole broader than those that survived. Weldon suggests 
that the frontal breadth is correlated with the breadth of the 
opening into the gill chamber, and the latter determines the 
amount of mud that enters, and in this way the elimination is 
produced. 
Should these conclusions of Weldon be established, they show 
that a change in the external conditions may cause the elimina- 
tion of certain individuals of the species, and in this way affect 
the mean of the survivors. In other words, natural selection 
occurs within the limits of the species. But it does not follow 
from this, as Weldon takes for granted, that if the same process 
of elimination were continued, a new species would be evolved. 
Raising the mean to the highest point attainable within the limits 
of the species does not necessarily lead to the formation of a new 
species. This point has already been discussed in other connec- 
tions. 
Crampton has studied variation and elimination in the moths 
of Philosamia Cynthia. In the first generation that he studied 
there was a high rate of mortality, and since the pupe were col- 
lected in their normal environment, before they had been sub- 
jected to the winter frost, the death rate must have been con- 
nected with some inherent weakness or with conditions that they 
met with during their normal life. Out of 942 pupe collected, 
there were 628 that were dead, so that only 329 ‘‘selected”’ 
individuals remained. The causes of the death of the 628 
pupe are not known. One might suspect that they had become 
parasitized ; but Professor Crampton tells me that he examined 
them to see if this was the case, and found no evidence of the 
sort. It does not seem probable that the external conditions 
to which the pupe were exposed had anything to do with the 
high death rate, and its causes are probably to be found in the 
organization of the animal. Whether bacterial disease, or dis- 
ease of some other kind, is responsible for the results, was not 
