ADAPTATION FOR THE SPECIES I45 
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. 
