14 



ANIMAL STUDIES 



cricket, a frog, and a kangaroo, because they all jump ; or 

 a bird, a bat, and a butterfly, because they all have wings 

 and can fly, although the different kinds of wings are made 

 in very unlike fashion. 



16. Natural classification based on homology. The closest 

 homologies are shown by those animals which have sprung 

 from a common stock. The basis of natural classification, 

 which is an expression of the ancestry of blood relationship 

 of animals, is therefore homology. So far as we know, the 

 actual presence of homologies among animals implies their 

 common descent from some stock possessing the same 

 characters. The close resemblance or homology among the 

 different races of men indicates that all men originally 



came from one stock. 

 As homology implies 

 blood-relationship, so, 

 on the other hand, 

 common descent im- 

 plies homology, the 

 similar parts being de- 

 rived from a common 

 ancestral stock. It is 

 sometimes said that 

 the inside of an animal 

 tells what it is, the out- 

 side where it has been. 

 In the internal struc- 

 ture, ancestral traits 

 are perpetuated with 

 little change through 

 long periods. The ex- 

 ternal characters, having more to do with surroundings, are 

 much more rapidly altered in response to demands of the 

 environment. 



A perfect classification would indicate the line of de- 

 scent of each member of the series, those now living 



FIG. 2. Wings showing homology and analogy, 

 a, fly ; b, bird ; c, bat. 



