EVOLUTION OF ANIMALS AND PLANTS 565 



of an animal or plant is not considered absolutely decided un- 

 til its embryology has been investigated and compared with 

 that of its allies. Animals and plants which are often highly 

 modified when adult are usually much like their near rela- 

 tives when in embryonic stages. By this means it has been 

 determined that some wingless insects are related to others 

 which have wings ; that the sac-like barnacle ( 316) and the 

 goose-barnacle are crustaceans; and so in hundreds of cases 

 study of embryos of animals and plants have disclosed hidden 

 resemblances 'which mean relationship. 



496. Geological Evidences of Evolution. If present ani- 

 mals and plants have descended from ancient ones, we ought 

 to find some evidence of changes in the fossils. This is 

 exactly what has been learned from study of the past history 

 of organisms preserved in the rocks. A vast number of 

 species of many groups of animals and plants have now been 

 collected and studied, and all the facts learned make biologists 

 more than ever convinced that the theory of evolution or 

 descent is true. One illustration must suffice for our present 

 study, and that is the fossils of ancestral horses showing 

 gradual reduction of toes to one on each foot which have been 

 found ( 361, 363). In the case of thousands of other species 

 of organisms, the great museums now contain fossils which 

 help in the study of relationships of both past and present 

 organisms. Popular books in this line are Lucas's " Ani- 

 mals of the Past " or Lancaster's " Extinct Animals." Simi- 

 lar popular books on plant fossils have not been written 

 because there is not such widespread knowledge and interest 

 in ancient plants as in the animals. However, the fossil 

 plants are to biologists no less interesting and convincing 

 than the animals. 



497. Distribution Evidences of Evolution. Briefly we 

 may state the central fact in this line as follows : Organisms 

 which appear in their similar structure and embryonic de- 

 velopment to be clearly related are commonly found in regions 



