464 ELEMENTARY BIOLOGY 



repeat themselves, generation after generation, in the same 

 sense as seasonal conditions of weather repeat themselves. 



487. Progressive changes. While much of the happening 

 in the universe is of a cyclic nature, there may nevertheless 

 be, for the world as a whole, a certain continuity of change 

 that has been called evolution. We need not feel called upon 

 to prove that all changes are altogether cyclic, or that all 

 changes are altogether evolutionary. It is quite reasonable and 

 consistent to recognize that both kinds of changes do actually 

 take place. 



488. Fossil evidences of evolution. How can we tell whether 

 the plants and animals of past times were different from those 

 of to-day ? 



The most direct evidence is furnished by the fossilized 

 remains of ancient plants and animals. Some two hundred 

 years ago people became interested in hard coal as a fuel ; 

 and in the digging of the coal, and in the digging into the 

 earth alongside of the coal seams, they came across structures 

 that in many ways suggested plant forms. Later they also found 

 stony structures that very decidedly suggested animal forms. 

 A study of these structures naturally led to an attempt to 

 classify them and to compare them with existing plants and 

 animals. These classifications lead to finding many resem- 

 blances between the organisms of the past and the organisms 

 of the present, but they also brought out marked differences. 

 Moreover, by arranging the series of fossils according to their 

 relative ages (which can be judged by their relative positions 

 in the layers of rocks) it was found possible in many cases to 

 show that the forms which were intermediate in age were also 

 intermediate in structure between the most ancient and the 

 most recent (Fig. 248). One of the best examples of this is 

 presented by the horse and his probable ancestors (see Fig. 249). 



Similar series of fossils have been worked out for the 

 elephant in Africa, for various fishes in England and else- 

 where, and for many lines of birds and reptiles in all parts of 



