PROBLEM 3. Explanation oj Some Fuzzling Facts 553 



ages. In breeding experiments we can 6, some puzzling cases of plant and ani- 



actually see changes going on. This idea mal distribution. 



that plants and animals change enables j^ .^^^ ^^ ^^^^^^ .^^ 



us to understand many puzzlmg facts. It ^^^^^ ^^^^ .^^^ ^^ ^^^^^ ^^ ^^^^^^^ 



helps explain such observations as these: ^ .u j c 1 r ' ^1 u 



r r strengthened, ouch tacts can then be 



1. the striking similarities in the skele- called evidences of the theory. Now the 

 ton and internal organs of vertebrates, idea or theory that new types arise from 



2. the great similarity in the develop- preceding types, the more complex 

 ment of vertebrate embryos. forms appearing as the descendants of 



3. the great chemical resemblances be- the simpler ones, so that all living things 

 tween certain animals and between bear a relationship to one another, is 

 certain plants. called the theory of organic evolution. 



4. the strange vestigial structures found The many puzzling facts explained in 

 in certain organisms. this problem can thus be called evidences 



5. the classification of organisms. of organic evolution. 



Questions 



1. What are the similarities in general skeletal structure of vertebrates 

 belonging to different classes (fish, amphibians, reptiles, birds, and 

 mammals)? What similarities do you note when you compare the 

 front appendages of a number of very different kinds of mammals: 

 Compare also the brains of members of the various vertebrate classes. 

 What do you discover? 



2. Define "anatomy" and "comparative anatomy." How do students of 

 anatomy explain similarities in vertebrate structure? 



3. In what respects do the early embryos of fish, frogs, reptiles, birds, and 

 mammals resemble one another? In which embryos do you find gill 

 slits and a tail? How do biologists now explain the striking resem- 

 blances of embryos? 



4. What examples of chemical resemblances can be explained by assum- 

 ing that organisms are related? 



5. What is meant by vestigial structures? Give three examples of vestigial 

 structures in man and one in the whale. 



6. Describe briefly the modern scheme of classification. Why is it that 

 animals of similar species can be placed in the same genus, animals of 

 similar genera always fit into one family, and so on all the way back 

 to the phylum? 



7. Give an example of interesting animal distribution. What barriers are 

 there to distribution? How can some cases of unusual distribution of 

 plants and animals be explained by biologists? 



8. Give a brief summary of this problem. 



