260 DEFINITE ACTION OF THE Chap. XXI1JL. 



CHAPTER XXIII. 



DIRECT AND DEFINITE ACTION OF THE EXTERNAL CONDITIONS OF 



LIFE. 



SLIGHT MODIFICATIONS IX PLANTS FROM THE DEFINITE ACTION' OF CHANGED 

 CONDITION'S, IN SIZE, COLOUR, CHEMICAL PROPERTIES, AND IN THE STATE 

 OF THE TISSUES — LOCAL DISEASES — CONSPICUOUS MODIFICATIONS FROM 



CHANGED CLIMATE OR FOOD, ETC. PLUMAGE OF BIRDS AFFECTED BY 



PECULIAR NUTRIMENT, AND BY THE INOCULATION OF POISON — LAND- 

 SHELLS — MODIFICATIONS OF ORGAN'IC BEINGS IN A STATE OF NATURE 

 THROUGH THE DEFINITE ACTION OF EXTERNAL CONDITIONS — COMPARISON 

 OF AMERICAN AXD EUROPEAN TREES — GALLS — EFFECTS OF PARASITIC 

 FUNGI — COXSIDE RATION'S OPPOSED TO THE BELIEF IX THE POTENT 

 INFLUENCE OF CHAXGED EXTERXAL COXD1TIOXS — PARALLEL SERIES OF 

 VARIETIES — AMOUNT OF VARIATION DOES XOT CORRESPOND WITH THE 

 DEGREE OF CHANGE IX THE CONDITIONS— RCD-VARIATION — MONSTROSI- 

 TIES PRODUCED BY UNNATURAL TREATMENT — SUMMARY. 



If we ask ourselves why this or that character has been modi- 

 fied under domestication, we arc. in most cases, lost in utter 

 darkness. Many naturalists, especially of the French school, 

 attribute every modification to the " monde ambiant," that is, 

 to changed climate, with all its diversities of heat and cold, 

 dampness and dryness, light and electricity, to the nature of 

 the soil, and to varied kinds and amount of food. By the 

 term definite action, as used in this chapter. I mean an action 

 of such a nature that, when many individuals of the same 

 variety are exposed during several generations to any par- 

 ticular change in their conditions of life, all, or nearly all the 

 individuals, are modified in the same manner. The effects of 

 habit, or of the increased use and disuse of various organs, 

 might have been included under this head ; but it will be con- 

 venient to discuss this subject in a separate chapter. By the 

 term indefinite action I mean an action which causes one in- 

 dividual to vary in one way and another individual in another 

 way, as we often see with plants and animals after they have 

 s i ejected for some generations to changed conditions of 

 life. But we know far too little of the causes and laws of 



