3 8 THE AGE OF THE EARTH 



When we look at this most important piece of research 

 as a whole, we cannot fail to be struck with the small 

 advance in insect structure which has taken place since 

 Carboniferous times. All the great questions of meta- 

 morphosis, and of the structures peculiar to insects, 

 appear to have been very much in the position in which 

 they are to-day. It is indeed probable enough that the 

 Orders which zoologists have always recognized as com- 

 paratively modern and specialized, such as the Lepido- 

 ptera, Coleoptera, and Hymenoptera, had not come into 

 existence. But as regards the emergence of the Class 

 from a single primitive group, as regards its approxima- 

 tion towards the Myriapods, which lived at the same 

 time, and of both towards their ancestor Peripatus, we 

 learn absolutely nothing. All we can say is that there 

 is evidence for the evolution of the most modern and 

 specialized members of the Class, and some slight pro- 

 gressive evolution in the rest. Such evolution is of 

 importance as giving us some vague conception of the 

 rate at which the process travels in this division of the 

 Arthropoda. If we look upon development as a series 

 of paths which, by successively uniting, at length meet 

 in a common point, then some conception of the position 

 of that distant centre may be gained by measuring the 

 angle of divergence and finding the number of unions 

 which occur in a given length. In this case, the amount 

 of approximation and union shown in the interval between 

 the Carboniferous Period and the present day is relatively 

 so small that it would require to be multiplied many 

 times before we could expect the lines to meet in the 

 common point, the ancestor of insects, to say nothing 

 of the far more distant past, in which the Tracheate 

 Arthropods met in an ancestor presenting many resem- 

 blances to Peripatus. But it must not be forgotten that 

 all this vast undefined period is required for the history 

 of one of the two grades of one of the three branches of 

 the whole Phylum. 



Turning now to the brief consideration of the second 

 grade of Arthropods, distinguished from the first grade 

 by the absence of antennae, the Trilobites are probably 



