VAKIATION. 269 



§ 90. Still there remains a difficulty. It may be said that 

 admitting functional change to be the initiator of variation 

 — granting that the physiological units of an organism, 

 modified by long subjection to new conditions, will tend to be- 

 come modified in such way as to cause change of structure m 

 offspring ; yet there will still be no cause of the »up])ose(i 

 heterogeneity among the physiological units of different in- 

 dividuals. There seems validity in the objection, that as all 

 the members of a species whose circumstances have been al- 

 tered, will be affected in the same manner, the results, when 

 they begin to show themselves in descendants, will show them- 

 selves 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 new requirement may be met by a 

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

 which the new region furnishes, mere accident may deter- 

 mine 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. I^ow when the functional divergences thus set 

 up in sundry families of a species, have lasted long euougb 



