234 EXPERIMENTAL EVOLUTION LECT. 



soil, from light to semi-darkness, from land to water, 

 &c. The experiment may be performed in a thousand 

 ways, and all external differences, all changes in 

 environment which seem to operate on organisms 

 may be successively tested. Care must be taken, 

 however, to watch the experiment without interfering, 

 and the animal or plant must be left wild, without 

 domestication or cultivation, so that they are exactly 

 in the condition of a species transferred from one set 

 of conditions to another without being particularly 

 helped to support the change. Other experiments 

 may be performed in a different manner. Instead of 

 altering slightly all conditions of environment as in 

 the preceding case, we may alter only one : for 

 instance, while keeping the animal or plant in its 

 native climate, alter the proportion of chemical com- 

 ponents of the soil, alter the nature of food, add some 

 new compound to the one or the other, increase or 

 decrease motion around it, &c. All the facts men- 

 tioned as illustrating the influence of environment 

 show how numerous and varied are the experiments 

 which may be performed, and it is needless therefore 

 to repeat what has already been said on the matter. 

 If the variations of environment have really effected 

 the result noticed, such result must be again obtained 

 by direct experiments. 



The fact is however, that in such experiments, the. 



