THE EVOLUTION OF LIFE 343 



One of the strongest indications that species are gene- 

 tically connected is furnished by the resemblance in struc- 

 ture which is found among the animals of any group. The 

 animals of any division of the animal kingdom are built 

 upon the same general plan of structure, however diverse 

 may be the modifications which they present. In the 

 mammals the limbs, for instance, are formed after much 

 the same pattern. In some cases bones may be fused 

 together that are separate in other animals, or certain 

 bones may be missing; but nevertheless it is possible by a 

 comparative study of limb structure to show how the 

 diverse forms may be derived from a common type. Organs 

 which are formed according to the same fundamental 

 pattern are called homologous, however diverse their form 

 and function. On the other hand organs which perform 

 the same function but which are different in their funda- 

 mental plan of structure are called analogous. Examples 

 of the first class are afforded by the arms of man, the fore 

 legs of a horse, the wings of bats and birds, and the flippers 

 of the whale. These organs have been modified in various 

 ways to subserve very different functions, but a study of 

 their structure shows them to bear a very close resemblance 

 nevertheless. Examples of analogous organs are afforded 

 by the wings of birds and the wings of insects which, while 

 they are both used as organs of flight, have very little 

 resemblance in structure. The resemblance of funda- 

 mental plan amid differences in the way in which the plan is 

 worked out is the natural result of inheritance from some 

 common ancestral form, the diversity being due to adapta- 

 tions to varied conditions during the divergence of species 

 from their common ancestor. 



Very striking indications of the descent of animals is 

 furnished by the existence of rudimentary organs. These 

 are organs of small size and degenerate structure, and they 



