122 THE LIVING WORLD. 



versally lost all except three pairs. Now the larvae 

 of butterflies (caterpillars) need a larger number of 

 legs than this, but instead of redeveloping the lost 

 legs they have simply had a fold of skin modified 

 to serve the function of legs. 



Early Forms Intermediate Between Existing Types. 



The next point of significance which we notice in 

 the examination of the history of animals is that, 

 although the earlier animals all conform to types still 

 in existence, it is frequently impossible to classify 

 them satisfactorily. Among the invertebrates there 

 is not very much difficulty in this respect, probably 

 because of the fact that even our earliest rocks 

 contained the types well differentiated from each 

 other. When we study the early representatives of 

 the higher types whose history is more or less com- 

 pletely represented in the early stages, it becomes 

 difficult and indeed impossible to determine where 

 to place them. The early insects of the Devonian 

 (3) and Carboniferous (4) seem to be related to sev- 

 eral of the present orders of insects, and it is impos- 

 sible to determine whether they are to be called 

 beetles, bugs, and dragon-flies, or are to be regarded 

 as all cockroaches showing an approach to these 

 other types of insects. Especially is this difficulty 

 of classification true in the case of the mammals. 

 Among the early representatives of this class of 

 animals in the Tertiary (9) some are so truly inter- 

 mediate in type that it is impossible to determine 

 whether they were insectivorans or marsupials, while 

 others -stand midway between the carnivores and 



