556 



THE CHANGING WORLD OF LIFE 



increases the rate of chromosome rearrangements which are an im- 

 portant part of evolution. By the use of radiation it is possible to 

 speed up evolution, so to speak, and study these changes first hand. 

 Then, by comparison of the chromosomes of closely related species it 

 is possible to trace the probable course of changes which have occurred 

 in the past. There is an interesting case to show how this happens in 

 nature. A group of rabbits were taken to the island of Porto Santo 

 during the fifteenth century. By the middle of the nineteenth century 

 their descendants had accumulated so many genie and chromosomal 

 changes that they were classified as a different species. 



» iSfe 



Photo by Winchester 



Fig. 34.13. Mesophippus, primitive ancestor of the modern horse, that lived about 30 

 million years ago. These horses were about the size of a collie dog and had three toes 

 on each foot. This is a reconstruction of three animals made after thorough study of 

 fossil remains of these animals. It is located in the Chicago Natural History Museum. 



Evidence from Zoogeography. Let us assume that a group of ani- 

 mals of the same kind are living in a temperate locality where there is 

 an average amount of rainfall and conditions are not extreme in any 

 form. They begin to spread out from this center to environments with 

 varying conditions. One group migrates into a hot, dry, desert-like 

 country ; another goes north and meets more severe winter weather ; an- 

 other moves south and ends up in a hot, humid jungle; others remain 

 where they are. Under such a variety of conditions, it is evident that 

 natural selection would proceed along different lines. After a long 



