ADAPTATION FOR THE SPECIES 145 



The fish may drop its eggs carelessly upon the bot- 

 tom of the stream. A frog may deposit them in a 

 mass of jelly and leave them forever. A turtle may 

 bury its eggs in a sand bank and abandon them to 

 their fate. The warm blood of the young bird de- 

 mands more attention than this. Accordingly, the 

 parent bird has learned to make for itself some sort 

 of nest, in which the young may be kept properly 

 warm until they are developed. The ancestral bird, 

 who was to be the progenitor of the entire bird class, 

 must have had some very simple method of providing 

 a place in which its eggs might be hatched. As the 

 descendants of this original bird have passed into new 

 situations, the various lines have taken upon them- 

 selves different shapes until we have the multiform 

 birds of to-day. The habits of the birds have also 

 varied. Each has adapted itself to the situation in 

 which it found itself, and no adaptation has been 

 more varied and effective than the adjustment of the 

 nesting site. Nests are found upon the ground, in 

 the bushes, on the lower limbs, in the crotches of the 

 trees, in the trunks of the trees, upon their very sum- 

 mits, and on the tops of inaccessible crags. To every 

 sort of situation some bird has been enabled to adapt 

 itself. This has made it possible for very many more 

 birds to thrive than could have found a place in the 

 world, had they all lived upon the same plan. 



