110 ORGANIC EVOLUTION CONSIDERED 



is, I think, small compared to that which had pre- 

 viously taken place. 



Taking the earliest representatives of the animal 

 kingdom and following them in their history, we find 

 a persistent tendency towards stability of form, and 

 little tendency to progress in organization. 



This incapacity to change, so marked among the 

 invertebrates of the Primordial, would lead us to 

 believe that au animal so different in structure and so 

 highly organized as a vertebrate could not have been 

 evolved from any invertebrate line. The more we 

 look at this class of facts the greater the difficulty 

 becomes. 



Insects and scorpions have been found in the 

 Silurian. They stand among the highest of even liv- 

 ing articulates, and they are the oldest known air- 

 breathing animals. 



We seek in vain for the progenitors of these highly 

 organized articulates or for some conceivable method 

 by which their wings and special breathing apparatus 

 could have been evolved. 



We do not know that these first insects and scor- 

 pions have made any material progress through all 

 the ages. As in the case of the Primordial animals, 

 so it is in this instance a remarkable fact that while 

 they must have progressed rapidly in order to become 

 insects and scorpions as early as the Silurian, yet 

 from that age to this they have made no advance in 

 structure. 



In the Devonian, May-flies have been found. In 

 the Carboniferous, several orders of insects are rep- 

 resented; the Neuroptera by Dragon-flies; the 

 Orthoptera by Locusts, Cockroaches, etc., and the 

 Coleoptera by Beetles. Spiders, Centipedes, and 

 Scorpions also existed. These various groups of ar- 



