THE GENESIS OF SPECIES. [Chap. 



descendants of a common stock, the modification being 

 greatest where the separation has been the most prolonged. 



"Rudimentary structures" also receive an explanation 

 by means of this theory. These structures are parts 

 which are apparently functionless and useless where they 

 occur, but which represent similar parts of large size and 

 functional importance in other animals. Examples of such 

 " rudimentary structures " are the foetal teeth of whales, 

 and those of the front part of the upper jaw of rumi- 

 nating quadrupeds. These foetal structures are minute in 

 size, and never cut the gum, but are re-absorbed without 

 ever coming into use, while no other teeth succeed or 

 represent them in the adult condition of those animals. 

 The mammary glands of all male beasts constitute another 

 example, as also does the wing of the apteryx — a New 

 Zealand bird utterly incapable of flight, and with the wing 

 in a quite rudimentary condition (whence the name of the 

 animal). Yet this rudimentaiy wing contains bones which 

 are miniature representatives of the ordinary wing-bones 

 of birds of flight. Now, the presence of these useless 

 bones and teeth is explained if they may be considered 

 as actually being the inherited diminished representatives 

 of parts of large size and functional importance in the 

 remote ancestors of these various animals. 



Again, the singular facts of homology are similarly 

 capable of deeper explanation by "Natural Selection." 

 " Homology " is the name applied to tlie investigation ol 

 those resenil»lances wliieh have so often been found to 

 underlie superficial ditferences between animals of very 

 ditYerent form and habit. Thus man, the horse, the whale, 

 and the bat, all have the pectoral limb, whether it be the 

 arm, or fore-leg, or paddle, or wing, formed on essentially 



