58 THE DESCENT OF MAN. 



many generations have become rudimentary and distorted, from 

 being continually rubbed and cliafed. We see tlie projecting part 

 in tliis condition in the Macacus brunneus, and absolutely aborted 

 in the M. ecaudatus and in several of the higher apes. Finally, 

 then, as far as ve can judge, the tail has disappeared in man and 

 the anthropomorphous apes, owing to the terminal portion having 

 been injured by friction during a long lapse of time; the basal 

 and embedded portion having been reduced and modified, so as to 

 become suitable to the erect or semi-erect position. 



I have now endeavored to show that some of the most dis- 

 tinctive characters of man have in all probability been acquired, 

 either directly, or more commonly indirectly, through natural se- 

 lection. We should bear in mind that 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 Geoffroy 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 in- 

 creased 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 wonderful growth of galls on plants caused by the poison of 

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

 age of parrots when fed on certain fishes, or inoculated 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 by Nageli on plants, and the remarks by 

 various authors with respect to animals, more especially those re- 

 cently made by Professor 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 confine my remarks to adaptive 

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

 during even the last few years, that very many structures which 



»= 'The Variation of Animals and Plants under Domestication,' vol. ii. 

 pp. 280, 282. 



