EVIDENCE FROM CLASSIFICATION 33 



animals with internal skeleton, which also includes 

 the fishes, amphibians, reptiles and birds. 



The principle in accordance with which these 

 groups of an ascending order of comprehensiveness 

 are constructed, is simply the greater or less degree 

 of likeness in structural characteristics. All mam- 

 mals agree in fundamental features, however much 

 they may differ in size, form, appearance and habits, 

 and thus such different-looking creatures as a whale, 

 a bat, a horse, a mole and a monkey are all referred 

 to the same class. Under the Linnsean conception, 

 which prevailed down to Darwin's time, the relation- 

 ship between the various species of a genus, or be- 

 tween the genera of a family, the families of an order, 

 etc., was regarded as purely ideal. If each species 

 represented a distinct act of creation and was so 

 fixed as to be incapable of transformation into an- 

 other species, then obviously no actual genetic re- 

 lationship, or blood-kinship between different species 

 was possible. According to the evolutionary hy- 

 pothesis, on the other hand, the relationship is truly 

 genetic and the differences are due to divergent de- 

 velopment, acting under different conditions and cir- 

 cumstances, while the deep-seated, fundamental like- 

 nesses are explained as inheritances from a common 

 ancestry. 



As Lamarck so emphatically pointed out, this dis- 

 tinction of species from one another is often exceed- 

 ingly difficult and has frequently given rise to great 

 differences of opinion and practice among systema- 



