CLASSIFICATION OF INSECTS. 55 



The generalized form of Thysanura, and the man- 

 ner in which it reappears in the larvae of other insects, 

 is the natural key of the classification, and will, we 

 hope, enable teachers to understand more clearly the 

 general relations of the orders. The characteristics 

 of adult insects are, as we shall have frequent occasion 

 to remark, often similar in widely separated groups 

 belonging to different orders, and such parallel or 

 representative repetitions have been the most fruitful 

 cause of the mis-association of forms in the older 

 classifications, not only among insects, but in all sub- 

 divisions of the animal kingdom. Since naturalists 

 have learned to use the hypothesis of evolution, great 

 changes have taken place in their estimate of the 

 value of the earlier stages of development. These 

 have been universally recognized as giving direct evi- 

 dence in their characteristics of the past history of 

 their own type. The changes of structure passed 

 through by the young during growth are more or less 

 transient, but, so far as they go, can be accurately de- 

 fined as abbreviated records-of the changes and modi- 

 fications which have been previously passed through 

 by the types to which they belong during their evolu- 

 tion in time. They have derived their principal char- 

 acteristics necessarily from their ancestors, and this 

 law of the correlation of the transient stages of the 

 young with the more permanent, specific, generic, or 

 type characters of the adults of ancestral generations, 

 or groups, is a necessary corollary of the law of evolu- 

 tion and heredity. Nevertheless, teachers must be 

 warned that the use of evidence of this kind requires 

 critical knowledge only acquired by long experience 



