152 THE WORLD OF LIFE chap. 



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 down 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 explained 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 

 marvellous developments of the vast world of life — birds 

 and insects — 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 

 which they have developed, suggest some very important 

 considerations. 



As we might expect, both birds and insects are com- 

 paratively rare in a fossil state, but there are sufficient 

 indications that the latter were first developed. A consider- 

 able number have been found in the Coal Measures, especi- 

 ally numerous cockroaches. Ancestral forms of Neuroptera 

 and Hemiptera allied to our may-flies and dragon -flies, 

 bugs and aphides, are found in Devonian and Carboniferous 

 rocks. The more highly 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 

 jaws, the celebrated Archaeopteryx. Diptera (flies) are also 

 found here, as well as a wasp, somewhat doubtfully identi- 

 fied ; while the most highly developed of all insects in 

 structure and metamorphosis, as well as in size and beauty, 

 the Lepidoptera, are first found in Tertiary beds, at a time 

 when birds allied to living forms also first appeared. 



This general parallelism of development seems clearly 



