I.] INTRODUCTORY. 19 



merous. Again, "Natural Selection" serves to explain 

 the circumstance that often in adjacent islands we find ani- 

 mals closely resembling, and appearing to represent, each 

 other ; while, if certain of these islands show signs (by 

 depth of surrounding sea or what not) of more ancient 

 separation, the animals inhabiting them exhibit a corre- 

 sponding divergence. 5 The explanation consists in rep- 

 resenting the forms inhabiting the islands as being the 

 modified descendants of a common stock, the modification 

 being greatest where the separation has been the most pro- 

 longed. 



" 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 func- 

 tional importance in other animals. Examples of such " ru- 

 dimentary structures" are the foetal teeth of whales, and 

 of the front part of the jaw of ruminating quadrupeds. 

 These foetal structures are minute in size, and never cut the 

 gum, but are reabsorbed without ever coming into use, 

 while no other teeth succeed them 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 condi- 

 tion (whence the name of the animal). Yet this rudiment- 

 ary wing contains bones which are miniature representa- 

 tives 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 func- 

 tional importance in the remote ancestors of these various 

 animals. 



5 For very interesting examples, see Mr. "Wallace's " Malay Archi- 

 pelago." 



