THE DEVELOPMENT OF MAN 323 



were not related, they would not repeat the structure of lower 

 types as they develop into their final form. 



3. Homologous Organs. In both plants and animals we find 

 parts, evidently of similar origin and structure, developed for very 

 different purposes. 



1. Leaves are modified into petals or thorns. 



2. Roots act as organs for climbing or storage. 



3. Hoofs, nails, and claws are all of similar origin. 



4. Scales, feathers, and hair are all modified forms of the same 

 epidermal structures. 



5. The various appendages of crayfish and its relatives are 

 evidently of similar structure, but modified to perform many 

 functions. 



Surely this modification of similar parts for different uses would 

 not be found if there were no relationship between the different 

 forms. 



4. Geological Evidence.. Although the fossil remains are neces- 

 sarily incomplete, still there have been found many series showing 

 gradual development from primitive to present forms. This is 

 notably true of the horse whose ancestors have been traced in 

 fossil skeletons back to a small five-toed form unlike any living 

 representatives. Also in the case of birds and reptiles, remains 

 have been discovered, showing plainly their descent from a common 

 ancestor. 



5. Domesticated Animals and Plants. We are continually 

 witnessing the development of different forms of plants and animals 

 in our methods of breeding, in which there is no question of relation- 

 ship of the new form to the old. 



Our many kinds of dog are descendants from the domesticated 

 wolf; the different breeds of hogs from the wild boar; fowls, 

 pigeons, sheep, and cattle, with their numerous breeds and 

 races, have been developed purposely by man, from very different 

 ancestors. 



From masses of such evidence, laboriously collected, all scientists 

 are agreed that all living things are related, the closeness being 

 indicated by the degree of similarity. They also agree that descent 



