EVOLUTION 241 



623. In most cases, individual plants may be assigned to very definite 

 species, and between these species transitional individuals are rarely or 

 never found. If species have been developed through a gradual evolu- 

 tion, why are such transitional forms absent? 



624. What characteristics must a successful weed possess? 



625. A weed introduced into a new region often becomes more wide- 

 spread and successful there than in its native land. Explain. 



626. The chestnut bark fungus, introduced some years ago into the 

 United States, has exterminated all the native American chestnut trees 

 over wide areas. In China, its native home, the species of chestnut are 

 almost immune to its attack. How do you explain this difference 

 between American and Chinese chestnut trees? 



627. Some species of plants produce comparatively few seeds but are 

 just as successful as others which produce a great many more. Explain. 



628. During the glacial invasion, the vegetation of the northern 

 United States was obUged to migrate hundreds of miles south of its 

 original range, and as the ice retreated it migrated northward again. 

 Doubtless many plant species were exterminated during these changes. 

 What characteristics should a plant species possess to survive such a 

 migration successfully? 



629. Name at least five different causes which might lead to the 

 extinction of a plant species. 



630. Why is it that all ancient and primitive types of plants have not 

 been exterminated by the competition of those which have been more 

 recently evolved? 



631. A highly specialized and complex plant species is sometimes far 

 less successful than one which is much simpler and more ancient in type. 

 Compare, for example, our common bracken fern, which thrives over 

 almost all the world, with many of our orchids, which are often rare 

 and have very limited ranges. How do you explain this? 



632. In the evolutionary history of many groups of animals and plants, 

 as shown by their fossils, there is a gradual change from the simple 

 and primitive members to those which are progressively more and more 

 complex and abundant; but when a very high degree of specialization has 

 arrived, the group suddenly becomes extinct. How do you explain this? 



633. Primitive and ancient types of animals and plants are most com- 

 mon in comparatively isolated regions. Why? 



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