VARIATION. 333 



admitting functional change to be the initiator of variation 

 granting that the physiological units of an organism long 

 subject to new conditions, will tend to become modified in 

 such way as to cause change of structure in offspring; yet 

 there will still be no cause of the supposed heterogeneity 

 among the physiological units of different individuals. There 

 seems validity in the objection, that as all the members of 

 a species whose circumstances have been altered will be affected 

 in the same manner, the results, when they begin to show 

 themselves in descendants, will show themselves in the same 

 manner: not multiform variations will arise, but deviations 

 all in one direction. 



The reply is simple. The members of a species thus cir- 

 cumstanced will not be similarly affected. In the absence of 

 absolute uniformity among them, the functional changes 

 caused in them will be more or less dissimilar. Just as men 

 of slightly-unlike dispositions behave in quite opposite ways 

 under the same circumstances; or just as men of slightly- 

 unlike constitutions get diverse disorders from the same 

 cause, and are diversely acted on by the same medicine; so, 

 the insensibly-differentiated members of a species whose con- 

 ditions have been changed, may at once begin to undergo 

 various kinds of functional changes. As we have already 

 seen, small initial contrasts may lead to large terminal con- 

 trasts. The intenser cold of the climate into which a species 

 has migrated, may cause in one individual increased con- 

 sumption of food to balance the greater loss of heat; while 

 in another individual the requirement may be met by a 

 thicker growth of fur. Or, when meeting with the new foods 

 which a new region furnishes, accident may determine one 

 member of the species to begin with one kind and another 

 member with another kind; and hence may arise established 

 habits in these respective members and their descendants. 

 Xow when the functional divergences thus set up in sundry 

 families of a species have lasted long enough to affect their 

 constitutions, and to modify somewhat the physiological units 



