NATURAL SELECTION. 97 



Youatt believes that the reduction of bone in some of our 

 sheep has already been carried so far that it entails great 

 delicacy of constitution. 



No doubt there is a limit beyond which 

 * se " ' the organization can not be modified compati- 

 bly with health or life. The extreme degree of fleetness, 

 for instance, of which a terrestrial animal is capable, 

 may have been acquired by our present race-horses ; but, 

 as Mr. Wallace has well remarked, the question that in- 

 terests us " is not whether indefinite and unlimited change 

 in any or all directions is possible, but whether such 

 differences as do occur in nature could have been pro- 

 duced by the accumulation of varieties by selection. " 

 And in the case of our domestic productions, there can 

 be no doubt that many parts of the organization, to 

 which man has attended, have been thus modified to a 

 greater degree than the corresponding parts in the natural 

 species of the same genera or even families. "We see this 

 in the form and size of our light and heavy dogs or 

 horses, in the beak and many other characters of our 

 pigeons, in the size and quality of many fruits, in com- 

 parison with the species belonging to the same natural 

 groups. 



HAS ORGANIZATION ADVANCED ? 



Origin of The problem whether organization on the 



Species, page whole has advanced is in many ways excess- 

 ively intricate. The geological record, at all 

 times imperfect, does not extend far enough back to 

 show with unmistakable clearness that within the known 

 history of the world organization has largely advanced. 

 Even at the present day, looking to members of the same 

 class, naturalists are not unanimous which forms ought 



