Chap. XXIII. DEFINITE ACTION OF CONDITIONS OF LIFE. 



271 



CHAPTER XXIII. 



DIRECT AND DEFINITE ACTION OF THE EXTERNAL CONDITIONS 



OF LIFE. 



SLIGHT MODIFICATIONS IN PLANTS FROM THE DEFINITE ACTION OF CHANGED CONDI- 

 TIONS, 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 ORGANIC 

 BEINGS IN A STATE OF NATURE THROUGH THE DEFINITE ACTION OF EXTERNAL CON- 

 DITIONS — COMPARISON OF AMERICAN AND EUROPEAN TREES — GALLS — EFFECTS 

 OF PARASITIC FUNGI — CONSIDERATIONS OPPOSED TO THE BELIEF IN THE POTENT 

 INFLUENCE OF CHANGED EXTERNAL CONDITIONS — PARALLEL SERIES OF VARIETIES 

 — AMOUNT OF VARIATION DOES NOT CORRESPOND WITH THE DEGREE OF CHANGE IN 

 THE CONDITIONS — BUD-VARIATION — MONSTROSITIES PRODUCED BY UNNATURAL 

 TREATMENT — SUMMARY. 



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

 fied under domestication, we are, 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 change in their physical 

 conditions of life, all, or nearly all the individuals, are modified 

 in the same manner. A new sub-variety would thus be pro- 

 duced without the aid of selection. 



I do not include under the term of definite action the effects 

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

 Modifications of this nature, no doubt, are definitely caused by 

 the conditions to which the beings are subjected; but they de- 

 pend much less on the nature of the conditions than on the laws 

 of growth; hence they are included under a distinct head in the 



