22 THE FACTORS OF OEQANIC EVOLUTION. 



&quot; The whole organization is so tied together during its growth and develop 

 ment, that when slight variations in any one part occur, and are accumu 

 lated through natural selection, other parts become modified.&quot; 

 And a parallel statement contained in Animals and Plants 

 under Domestication, vol. ii, p. 320, runs thus 

 V/ ^Correlated variation is an important subject for us ; for when one part 

 is modified through continued selection, either by man or under nature, 

 other parts of the organization will be unavoidably modified. From this 

 correlation it apparently follows that, with our domesticated animals and 

 plants, varieties rarely or never differ from each other by some single 

 character alone.&quot; 



v/ By what process does a changed part modify other 

 parts ? By modifying their functions in some way or 

 degree, seems the necessary answer. It is indeed, imagin 

 able, that where the part changed is some dermal appen 

 dage which, becoming larger, has abstracted more of the 

 needful material from the general stock, the effect may 

 consist simply in diminishing the amount of this material 

 available for other dermal appendages, leading to diminu 

 tion of some or all of them, and may fail to affect in 

 appreciable ways the rest of the organism : save perhaps 

 the blood-vessels near the enlarged appendage. But where 

 the part is an active one a limb, or viscus, or any organ 

 which constantly demands blood, produces waste matter, 

 secretes, or absorbs then all the other active organs 

 become implicated in the change. The functions per 

 formed by them have to constitute a moving equilibrium ; 

 and the function of one cannot, by alteration of the struc 

 ture performing it, be modified in degree or kind, without 

 modifying the functions of the rest some appreciably and 

 others inappreciably, according to the directness or indi 

 rectness of their relations. Of such inter-dependent changes, 1 / 

 the normal ones are naturally inconspicuous; but those 

 which are partially or completely abnormal, sufficiently 

 carry home the general truth. Thus, unusual cerebral 

 excitement affects the excretion through the kidneys in 

 quantity or quality or both. Strong emotions of disagree 

 able kinds check or arrest the flow of bile. A considerable 



