THE INFLUENCES OF ENVIRONMENT 277 



numerous and excellent observations of the plant physiologists. That 

 all these are adaptations and secondary reactions to stimuli is proved 

 by the fact that the same stimuli affect the homologous parts of 

 different species in very different, and often in opposite ways. For 

 instance, while the green shoots of most plants turn towards the light, 

 being positively heliotropic, the climbing shoots of the ivy and the 

 gourd are negatively heliotropic, which is an adaptation to climbing. 

 In this case the reason of the difference in the mode of reaction nmst 

 lie in the difference of constitution of the cellular substance of the 

 shoot, and since this may differentiate so very diversely in its relation 

 to light, the power of reaction which plant substance in general has 

 to light must not be regarded as a primary character, like the specific 

 gravity of a metal or the chemical affinities of oxygen and hydrogen, 

 but as adaptations of the living and varying substance to the special 

 conditions of life. And the origin of these adaptations must depend 

 upon processes of selection, and on these alone. This is just the 

 difference between living and non-living matter, — that the former 

 is variable to a high degree, the latter is not ; it is the fundamental 

 difference upon which the whole possibility of the origin of an animate 

 world depends. 



Among animals also we must distinguish between the direct 

 effects of external influence to which the organism is not already 

 adapted, and those reactions which imply a previousl}^ established 

 adjustment to the stimulus. That is, we must distinguisli between 

 primary and secondary reactions. 



For instance, Herbst made artificial sea- water in which the 

 sodium was partially replaced by lithium, and the eggs of sea-urchins 

 developed in this artificial sea-water into very divergent larvae of 

 peculiar structure. We have here a primary reaction of the organism 

 to changed conditions of life— not an adaptation, not a prepared 

 reaction. Accordingly these ' lithium larva? ' eventually perished. 



The increasing blackness of Polyommatiis phlwas, which we have 

 already discussed, must also be regarded as a primary reaction, l)ut 

 not so the variations — often misinterpreted— of those species of 

 Artemia which live in the brine-pools of the Crimea, in regard 

 to which Schmankewitsch showed that, when the amount of salt 

 in the water is diminished, they undergo certain changes wliich bring 

 them nearer to the fresh -water form Branchqms, while when the salt 

 is increased in amount they var}^ in the contrary direction. Probably 

 these are adaptations to the periodically changing salinity of then* 

 habitat. 



There can be no doubt of this in the case of the caterpillars 



