VARIATION. 2G9 



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 in 

 offspring ; yet there will still be no cause of the supposed 

 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- 

 urilike 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. Now when the functional divergences thus set 

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



