XIV 
FUNDAMENTAL PROBLEMS 
427 
immense body of interesting facts showing the influence of 
food, of light, of temperature, of still water and moving water, 
of the atmosphere and its currents, of gravitation, and of other 
organisms, in modifying the forms and other characteristics of 
animals. 1 He believes that these various influences produce 
a direct and important effect, and that this effect is accumu¬ 
lated by inheritance; yet he acknowledges that we have no 
direct evidence of this, and there is hardly a single case 
adduced in the book which is not equally well explained by 
adaptation, brought about by the survival of beneficial varia¬ 
tions. Perhaps the most remarkable case he has brought 
forward is that of the transformation of species of crustaceans 
by a change in the saltness of the water (see Fig. 35). Artemia 
salina lives in brackish water, while A. Milhausenii inhabits 
water Avhich is much salter. They differ greatly in the form of 
the tail-lobes, and in the presence or absence of spines upon the 
tail, and had always been considered perfectly distinct species. 
Yet either was transformed into the other in a few generations, 
during which the saltness of the water was gradually altered. 
Yet more, A. salina was gradually accustomed to fresher 
water, and in the course of a few generations, when the water 
had become perfectly fresh, the species 
was changed into Branchipus stag- 
nalis, which had always been con¬ 
sidered to belong to a different genus 
on account of differences in the form 
of the antenme and of the posterior 
segments of the body (see Fig. 36). 
This certainly appears to be a proof 
of change of conditions producing 
a change of form independently of 
selection, and of that change of form, 
while remaining under the same con¬ 
ditions, being inherited. Yet there 
is this peculiarity in the case, that 
there is a chemical change in the water, and that this water 
permeates the whole body, and must be absorbed by the 
tissues, and thus affect the ova and even the reproductive 
1 The Natural Conditions of Existence as then Affect Animal Life. 
London, 1883. 
