174 A HISTORY OF BIRDS 



with the increasing perfection of their procreant cradles. As 

 the following facts will show, nidification, in its broad outlines 

 at least, is determined by environment, though there can be no 

 doubt but that other factors are at work, some of which perhaps 

 we can at least indicate while others yet remain to be discovered. 



That the earliest birds were arboreal there can be little room 

 for doubt, and we may assume that they brooded their eggs 

 either in the holes of trees or on the stumps of decaying tree- 

 ferns, or amid the crowns of evergreen oaks, and similar trees 

 which had, with the appearance of the first bird — Archaeopteryx 

 — already come into existence. It may be objected how- 

 ever, that the long and peculiarly formed tail of Archaeopteryx 

 was but ill-suited to the cramped quarters of a hollow tree-trunk 

 — though the long-tailed modern Hornbills contrive to overcome 

 the diiificulty — and that the work of hatching was carried on in 

 the open. But be this as it may, with the gradual spread of 

 the race some became denizens of the open country, and these 

 would probably at first have deposited their eggs on the bare 

 ground without making any special preparations for their safety 

 or protection. Two new selective factors would now come into 

 operation, one tending to eliminate all eggs which were not 

 protectively coloured (p. 206), and on the other, all such as 

 suffered from contact with cold or moist earth. It is not diffi- 

 cult to imagine that sooner or later more or fewer of the birds 

 nesting in such sites would hit upon the fllan of collecting bits 

 of grass and stick or small stones into a small heap whereon to 

 lay their eggs, prompted not so much by any conscious desire 

 to protect the eggs from injury as to keep warm and dry when 

 sitting where the ground was damp. Only those birds which 

 had sufficient intelligence to adopt this expedient would rear 

 offspring, and this offspring would probably inherit the same 

 instinct. Thus were the first nests built. The habit of building 

 a nest once fixed, wherever the eggs were laid, some receptacle 

 would be first constructed, and thus the way was prepared for 

 those birds which, to avoid enemies, took to laying their eggs 

 amid the branches of shrubs and trees. 



The possibility that the earliest nesting sites were holes in 

 trees receives some little support from the fact that many birds 

 still retain this habit, and lay white eggs (p. 208). As the 

 primitive, arboreal bird left the forest regions some sought the 



