164 THE WORLD OF LIFE 



the attacks of birds-of-prey and other enemies. Hence the 

 earliest flocks will have a larger proportion of young birds than 

 the later ones. The earlier flocks also, being less pressed for 

 time will be able to choose fine weather for the crossing, and 

 thus it will be only the young and quickly-fatigued birds that 

 will probably fly low and come dow^n to rest. Later on every 

 recurrence of bad weather will drive down old and young alike 

 for temporary shelter and rest. Thus all the facts are ex- 

 plained without having recourse to the wildly improbable 

 hypothesis of flocks of immature birds migrating over land 

 and sea quite alone, and a week in advance of their parents 

 or guides. 



What this World-wide Adaptation teaches us 



This co-adaptation of two of the highest and most marvel- 

 lous developments of the vast world of life — birds and in- 

 sects — an adaptation which in various forms pervades all 

 their manifestations upon the earth, from the snow^-wastes of 

 the tundra to the glorious equatorial forests; and the further 

 co-adaptation of both, with the vegetation amid w'hich they 

 have developed, suggest some very important considerations. 



As we might expect, both birds and insects are comparatively 

 rare in a fossil state, but there are suflicient indications that 

 the latter were first developed. A considerable number have 

 been found in the Coal Measures, especially numerous cock- 

 roaches. Ancestral forms of ^N^europtera and Hemiptera allied 

 to our may-flies and dragon-flies, bugs and aphides, are found 

 in Devonian and Carboniferous rocks. The more his^hlv 

 organised insects with a complete metamorphosis, come later; 

 beetles, dragon-flies, and bugs (Hemiptera) are rather common 

 in Lias beds, and here, for the first time, we meet with a 

 true ancestral bird with perfectly developed wings and 

 feathers, and with toothed jaw^s, the celebrated Archseopteryx. 

 Diptera (flies) are also found here, as ^vell as a wasp, some- 

 what doubtfully identified ; while the most highly developed 

 of all insects in structure and metamorphosis, as well as in 



