THE STRUCTURE AND HABITS OF ANIMALS 29 



of my paper, viz., the influence of environment on 

 the structure and habits of animals. It will, of 

 course, be at once conceded that changes in environ- 

 ment not only can but do produce changes of very 

 great magnitude in both the structure and habits of 

 animals for example, domestic breeds of cattle, 

 &c.; but in most of these cases the conditions are 

 very complicated, and it is impossible to assign to 

 each factor its true value. What I wish to prove 

 to you, if possible, is that change of environment 

 not only produces changes of structure and habit, 

 but also produces definite and calculable changes, 

 so that a definite change in environment is always 

 followed by a definite change of structure, and the 

 result can be predicted with as much certainty as a 

 chemical reaction. 



For this purpose all ordinary examples fail us 

 because the environment is too complicated, e.g., 

 pigeon fanciers can produce at pleasure a bird with 

 any number of feathers in the tail they wish, and 

 with almost any variety of plumage, but as to which 

 of the elements of a complicated and artificial en- 

 vironment the result is due we are completely in the 

 dark. For this purpose it is necessary to choose 

 our examples carefully, and the number of available 

 ones that are not of too technical a character for 

 general discussion is very limited. 



We have seen that it is very easy to establish a 

 general relation between changes of environment 

 and changes of structure, and that the one series is 

 followed at once by the other. Now this general 

 relation is not sufficient for our proposition, much 



