How Animals Work. 



we may assume that they brooded their eggs in the holes 

 of trees or in 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. 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, and the other all such as suffered from con- 

 tact with cold or moist earth. It is not difficult to 

 imagine that, sooner or later, more or fewer of the birds 

 nesting in such sites would hit upon the plan of collect- 

 ing bits of grass and sticks 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 off- 

 spring, 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. As the primitive arbo- 

 real bird left the forest regions, some sought the dark 



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