10 ZOOLOG Y. 



the form of the teeth presupposes either a ruminant or carni- 

 vore. Hence this prime law of comparative anatomy led to 

 the establishment by Ouvier of the fundamental laws of 

 palaeontology, by which the comparative anatomist is en- 

 abled to restore from isolated teeth or bones the probable 

 form of the original possessor. Of course the more perfect 

 the series of bones and teeth, or the more complete the re- 

 mains of insects or mollusks, the more perfect will be our 

 knowledge, and the less room will there be for error in re- 

 storing extinct animals. 



Adaptation. An organ with a certain normal use or 

 function may be adapted, in consequence of a change in the 

 habits of the animal, to another use than the original one. 

 To take an extreme case, the Anabas, or climbing fish, may 

 use its fins to aid it in ascending trees. On the other hand, 

 by disuse organs become aborted or rudimentary. The 

 teeth of the whalebone whale are rudimentary in the young, 

 and are replaced by whalebone, which is more useful to the 

 animal ; the eyes of the blind-fish are rudimentary, func- 

 tionless. Those of certain cave-insects are entirely wanting, 

 being lost through disuse, owing to a change of life from 

 the light, outer world to totally dark caverns, and the con- 

 sequent disuse of their eyes. Nature is economical. Every 

 thing that is not of use as a rule disappears. It would be a 

 waste of material to nourish and care for an organ in a cave- 

 animal, or a parasitic insect or crustacean, which would be 

 of no use to the animal. On the other hand, if the leg or 

 tail of a newt is snipped off by some rapacious fish, it 

 grows out again. 



Moreover, the animal organism is far more pliable than is 

 generally supposed. Not only is nature continually repair- 

 ing wounds and waste, not only is the body being contin- 

 ually made over again, but certain animals undergo a 

 change of form, either generally or in particular parts. If 

 the environment is unchanged, the animal remains true to 

 its species. The dogma of the invariability or stability of 

 species is a fallacy. Change the climate, moisture or dryness, 

 the nature of the soil ; introduce the natural enemies of the 

 animal or remove them ; destroy the balance of nature, in 



