30 A BOOK OF INSECTS 



things, in all their endless variety, are the outcome of 

 special creation, has given place to the more plausible 

 theory that the existing species of plants and animals 

 were derived, through a slow process of modification, 

 from more simple forms. Many of the older naturalists 

 believed that this relationship of living things was in 

 the nature of a progressive scale, like the rounds of a 

 ladder. They put the simplest plants and animals at 

 the base, and added the remaining groups, step by step, 

 in the order of their increasing complexity. But advancing 

 science has shown that the true affinity of living things 

 cannot thus be expressed. On the contrary, a careful 

 survey of all existing species, and of extinct species which 

 are known to us as fossils, suggests that their relationship 

 may be most fittingly compared to the branching of 

 a tree towering upwards from the root-stock of life. 

 Each of the great branches is termed a phylum, or tribe. 

 One, in the case of animals, comprises the vertebrates, 

 another the cuttles and shell-fishes, and so on ; while the 

 phylum with which we are specially concerned includes 

 the annulate or ringed worms, wheel animalcules, spiders, 

 crabs, centipedes and insects. 



We must realise that the creatures comprised in each 

 tribe, or phylum, are all, so to say, variants of one root 

 idea. In other words, they are all built up from a single 

 plan, simple in itself, but capable of endless modification 

 and improvement. This plan, in the case of the phylum 

 to which insects belong, consists of a head-lobe followed 

 by a series of nearly identical rings, or segments, each 

 enclosing a portion of the body-cavity, with its share 

 of the nerve- cord, the digestive tract and the main blood- 

 vessels. Each segment also carries a pair of external 

 appendages, or limbs, symmetrically arranged, with their 

 necessary motor muscles. Hence these creatures are 



