100 GENERAL ZOOLOGY 



does not give us much help in this particular problem. How- 

 ever, of the remains of winged insects which have so far been 

 discovered, the earliest are those belonging to the orders 

 Orthoptera, Hemiptera, Plectoptera, and Odonata. 



A third source of information comes from the study of the 

 earlier, or embryological, stages in the development of the 

 individual. According to von Baer, a Russo-German natural- 

 ist born in 1792, the more nearly alike two animals belonging 

 to the same phylum are, the greater will be the resemblance 

 in their embryological stages ; that is, the longer will the 

 animals continue to follow a similar line of development. 

 This is known as von Baer's law. Since his time the theory 

 has been advanced that each individual animal, being the 

 product of its ancestors, reproduces to a greater or less extent 

 the stages which have occurred in the history of the race. 

 According to this view the earlier stages in the history of 

 the individual represent adult stages in the life of the past. 

 This is known as the recapitulation theory. This principle 

 applied to the cockroaches (see p. 20), would indicate their 

 descent from ancestors which were more cylindrical than the 

 flattened forms of to-day. 



The conclusions of embryology justify the general state- 

 ments just made, so that it is possible to assert that the orders 

 without strongly marked metamorphosis are, on the whole, the 

 " lower," or more generalized, insects ; while the orders with 

 marked (" complete") metamorphosis represent the "highest," 

 or most specialized, types of the class to-day. The sequence 

 in which the different orders of insects have been described 

 in these pages represents, on the whole, with the excep- 

 tion of the Thysanura, a gradually ascending series from the 

 more generalized forms to the highly specialized Lepidoptera, 

 Diptera, and Hymenoptera. 



