THE FACTORS OF EVOLUTION 189 



and that the pupae of the butterflies would be exposed during their 

 development to such a cold temperature as that to which they 

 were subjected in Fischer's experiments, then the same causes 

 would, according to our knowledge, produce the same results in 

 nature. We should see species of butterflies exhibiting modifi- 

 cation in their colour towards the dark shade. As the descen- 

 dants of these modified butterflies would again be exposed to 

 the same conditions, they would possess the same charac- 

 teristics as their parents, no matter whether these newly- 

 acquired characteristics had been transmitted by their parents 

 or not. 



It is clear that the change in the conditions of life 

 exercises an influence not only upon colour, but also upon size, 

 scale formation, etc. In course of time, therefore, animals would 

 come into existence so widely differentiated from their ancestors 

 that we should regard them as new species. The original 

 species, however, would in the meantime have become extinct. 

 Whether the characters of these new species would be 

 really constant and able to maintain themselves in the event 

 of the reconstruction of the original conditions does not affect 

 the question which could, indeed, only arise if some one would 

 be able artificially to reproduce the old conditions. If it were 

 possible to sow seeds of different plants of our earth on any 

 suitable star in the Universe, the plants that grew from those 

 seeds, would probably be so changed that we should be unable 

 to recognize them. 



How radical the changes can be which are produced in an 

 organism by an artificial modification of the normal conditions 

 of life will require further attention. It is not saying too 

 much when we describe animals and plants as compulsory forms 

 dependent in appearance and structure upon the stimuli which 

 have influenced them during their life. There exists, as it were, a 

 fluctuating balance between the organisms and the external con- 

 ditions of life : a change in the stimuli calls forth corresponding 

 changes in the organization. The simpler an organism, the less 

 resistance it seems generally able to offer to changes. From 

 numerous facts one gains further the impression that the length 

 of the action of a stimulus has a considerable influence upon 

 the constancy or inconstancy of a certain modification. 



