THE HAUNTS OF LIFE 125 



From the evolutionist point of view it is interesting 

 to notice that in Bird, Bat and Pterodactyl the flying 

 organ is in each case the arm, and yet the details of the 

 transformation are very different in the three cases. With 

 precisely the same fundamental material to work with, 

 three entirely different types of wing have been evolved. 

 In insects the wings appear to be entirely novel struc- 

 tures — hollow, flattened sacs growing out from the upper 

 parts of the two posterior divisions of the thorax ; but it 

 is possible that they were, to begin with, rather respiratory 

 than locomotor organs. Indeed, in some cases they still 

 have considerable respiratory function — containing blood- 

 channels and extensions of the air-tubes or tracheae. As 

 illustrations of analogy it is interesting to compare Birds 

 and Insects, for they are as far apart from one another 

 anatomically as they could well be, and yet they have 

 much in common — lightly built bodies, highly speciahzed 

 musculature, very elaborate respiratory system with active 

 expiratory movements, and so on. These are convergent 

 adaptations towards the same end in entirely different 

 types. 



It is probable that the Vertebrate animals which have 

 attained to the power of true flight have sprung from 

 arboreal stocks. It is likely that the oldest known bird 

 — the extinct Archaeopteryx— which had teeth on both 

 jaws, a long lizard-Hke tail, and claws on each of the three 

 digits of the half-made wing, was definitely arboreal. The 

 same conclusion is suggested by the Hoatzin {Opistho 

 comus), one of the most primitive of living birds, whose 

 young ones clamber about on the branches. It is probable 

 that the Bats sprang from a stock of arboreal Insectivores. 



Most of the insects which are aerial as adults spend the 



