98 LAWS OF VARIATION. [CHAP. V, 



limbs divided into great branches, and these into lesser and lesser 

 branches, were themselves once, when the tree was young, budding 

 twigs; and this connection of the former and present buds by 

 ramifying branches may well represent the classification of all 

 extinct and living species in groups subordinate to groups. Of 

 the many twigs which flourished when the tree was a mere bush, 

 only two or three, now grown into great branches, yet survive and 

 bear the other branches ; so with the species which lived during 

 long-past geological periods, very few have left living and modi- 

 iied descendants. From the first growth of the tree, many a limb 

 and branch has decayed and dropped off; and these fallen branches 

 of various sizes may represent those whole orders, families, and 

 genera which have now no living representatives, and which are 

 known to us only in a fossil state. As we here and there see a 

 thin straggling branch springing from a fork low down in a tree, 

 and which by some chance has been favoured and is still alive on 

 its summit, so we occasionally see an animal like the Ornitho- 

 rhynchus or Lepidosiren, which in some small degree connects by 

 its affinities two large branches of life, and which has apparently 

 been saved from fatal competition by having inhabited a protected 

 station. As buds give rise by growth to fresh buds, and these, if 

 vigorous, branch out and overtop on all sides many a feebler 

 branc ;, so by generation I believe it has been with the great Tree 

 of Life, which fills with its dead and broken branches the crust of 

 the earth, and covers the surface with its ever-branching and 

 beautiful ramifications. 



CHAPTER V. 

 LAWS OP VARIATION. 



Effects of changed conditions Use and disuse, combined with natural selection ; 

 organs of flight and of vision Acclimatisation Correlated variation 

 Compensation and economy of growth False correlations Multiple, 

 rudimentary, and lowly organised structures variable Parts developed in 

 an unusual manner are highly variable : specific characters more variable 

 than generic : secondary sexual characters variable Species of the same 



fenus vary in an analogous manner Eeversions to long-lost characters 

 ummary. 



I HAVE hitherto sometimes spoken as if the variations so common 

 and multiform with organic beings under domestication, and in a 

 lesser degree with those under nature were due to chance. This, 

 of course, is a wholly incorrect expression, but it serves to acknow- 



