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 which 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 antennse 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 they Affect Animal Life. 

 London, 1883. 



Fig. 36. 

 Branchipus stagnalis. 

 b. Artemia salina. 



