o» THE DESCENT OF MAN 



modifications in structure or constitution which do not serve 

 to adapt an organism to its habits of life, to the food which it 

 consumes, or passively to the surrounding conditions, cannot 

 have been thus acquired. We must not, however, be too 

 confident in deciding what modifications are of service to 

 each being: we should remember how little we know about 

 the use of many parts, or what changes in the blood or 

 tissues may serve to fit an organism for a new climate 

 or new kinds of food. Nor must we forget the principle 

 of correlation, by which, as Isidore Geofiroy has shown in 

 the case of man, many strange deviations of structure are 

 tied together. Independently of correlation, a change in 

 one part often leads, through the increased or decreased 

 use of other parts, to other changes of a quite unexpected 

 nature. It is also well to reflect on such facts as the won- 

 derful growth of galls on plants caused by the poison of 

 an insect, and on the remarkable changes of color in the 

 plumage of parrots when fed on certain fishes, or inocu- 

 lated with the poison of toads;" for we can thus see that 

 the fluids of the system, if altered for some special purpose, 

 might induce other changes. We should especially bear 

 in mind that modifications acquired and continually used 

 during past ages for some useful purpose would probably 

 become firmly fixed, and might be long inherited. 



Thus a large yet undefined extension may safely be given 

 to the direct and indirect results of natural selection; but I 

 now admit, after reading the essay of Nageli on plants, and 

 the remarks by various authors with respect to animals, 

 more especially those recently made by Prof. Broca, that 

 in the earlier editions of my "Origin of Species" I perhaps 

 attributed too much to the action of natural selection or the 

 survival of the fittest. I have altered the fifth edition of 

 the "Origin" so as to (pnfine my remarks to adaptive 

 changes of structure; but I am convinced, from the light 

 gained during even i;he last few years, that vwy many 



96 "The Variation of Animals and Plants under Domestication,'' vol ii. pp^ 

 280, 282. 



