40 ILLINOIS STATE ACADEMY OF SCIENCE 



birds or mammals; and among seed plants the Gymno- 

 sperms appeared before the higher Angiosperms. 



2. The connecting or transitional characters pos-' 

 sessed by the earliest representatives of any class of 

 plants or animals present still more definite evidence of 

 evolution. For example, the earliest birds are found in 

 rocks of Jurassic age. These first bird forms had teeth 

 in both lower and upper jaws, like reptiles, a long verte- 

 brated tail, like reptiles, and, like reptiles had separate 

 toes, ending in claws, on their front limbs or wings. In 

 fact they show so clearly their reptilian relationship 

 that if it were not for the feathers with which these birds 

 were scantily clothed, there would be no hesitation in 

 calling them reptiles. 



A somewhat different kind of connecting or transi- 

 tional forms is shown in the classic example of the evo- 

 lution of the horse, of which a most complete series of 

 skeletons has been found in rocks ranging from Eocene 

 to Pliocene in age. These show every step in the change 

 from the small Eocene horse, about as large as a fox 

 terrier, and having four toes and a rudiment of another 

 toe on each front foot, and three toes and a splint on 

 each hind foot, to the full size modern horse found in 

 late Pliocene rocks, having one functional toe and two 

 splints or rudiments of other toes on each foot. The evo- 

 lution of the elephant and camels is known by series of 

 skeletons almost as complete as that of the horse. 



3. The evidence of evolution shown by the law of 

 recapitulation is possibly even more conclusive than 

 that already cited. This law states that the life history 

 of each individual recapitulates, or repeats in a short- 

 ened way, the evolutionary history of the race to which 

 it belongs. A clear illustration of this law is shown in 

 the life history of the frog, the young stage of which 

 is a tadpole having no lungs or legs, but breathes by 

 means of gills, swims by movements of its tail, and is 

 a fish in all its main characteristics and habits. Later it 

 develops legs and lungs ; absorbs its gills and tail ; leaves 

 the water, and is adapted to life on land. According to 

 the law of recapitulation, the fish stage in the early life 

 of the frog indicates a fish ancestry for the class Am- 



