PROBLEM 3. Explanation of Some Puzzling Facts 



551 



Fig. 504 Skeleto?z of a por- 

 poise, relative of the whale, 

 which never leaves the 

 water. What vestigial struc- 

 ture do you see? (ward's 



NATURAL SCIENCE ESTABLISH- 

 MENT) 



bones on each side of the spinal col- 

 umn in that region where in most other 

 mammals the hind legs would be at- 

 tached. These are but a few of many 

 examples of apparently useless structures 

 called vestigial (ves-tij'ee-al) structures. 

 These structures seem to be remnants or 

 vestiges of structures that in related ani- 

 mals are useful. It is believed that the 

 bones in the whale are vestiges of the 

 bones possessed by the whale's distant 

 ancestors, animals which had and used 

 hind legs. So with the appendix in man. 

 In rabbits and in some other mammals 

 the appendix is a large organ used in 

 digestion. All such seemingly useless 

 organs are puzzling unless we assume 

 that the different tA'pes of animals are 

 descended from some distant common 

 ancestor or ancestors. 



Resemblances and classification. You 

 know that we classify plants and animals 

 according to their resemblances in struc- 

 ture. All those that have in common a 

 few striking structures are placed to- 

 gether in a large group known as a 

 phylum. For example, animals with a 

 hard outside covering, segmented bodies, 

 and jointed legs are placed together in 

 the arthropod phylum. The animals of 

 each phylum are then separated into 

 different classes according to some other 

 definite resemblance. Those within a 

 class are again subdivided into orders 



according to other similar structures; 

 those within each order are subdivided 

 into different families, those within each 

 family into genera, and lastly those 

 within each genus are put into different 

 species. As a result the animals within a 

 species are similar in most of their struc- 

 tures. The animals of two different spe- 

 cies within one genus are alike in fewer 

 of their structures. Animals of two dif- 

 ferent genera within one family have 

 even less in common, and so on. At first 

 it seemed astonishing that plants and ani- 

 mals should fit so neatly into this scheme 

 of classification. But remember that we 

 believe that fossils seem to show that 

 animals and plants changed step by step 

 throughout the ages. If this is true, they 

 are descended one from another and are 

 related, some more closely, some more 

 distantly. Then it becomes clear why 

 some have a great deal in common, 

 others have less in common. And if this 

 is true they will naturally fit into a phy- 

 lum which is subdivided into classes, 

 then into orders, into families, into 

 genera and species. Now do Exercises 

 4 and 5. 



(Optional) Distribution of plants and 

 animals. In general, animals and plants 

 tend to spread far and wide over the 

 earth to areas which are suitable in cli- 

 mate, provide the right food in sufficient 

 amounts, and have not already beer 



