BRITISH WARBLERS 



what exactly do we mean ? That the initial efforts of the 

 young bird to rear its offspring are doomed to failure? 

 Hardly, one would think, if the preservation of the race is to 

 be secured. The nest has a very definite biological end to 

 serve ; it matters not whether it is a little more or a little less 

 highly elaborated; what matters is that it should be 

 sufficiently stable to fulfil the requirements of incubation and 

 ensure the safety of the offspring. Those individuals that 

 failed in the first attempt, even though their failure was 

 tempered by success on subsequent occasions, could have no 

 chance with others in whom the instinct was perfect from 

 birth, and must have long since been eliminated. Moreover 

 no increase of skill begotten of experience, which would lead 

 to a greater perfection of detail, a more highly elaborated 

 production over and above that which was necessary for 

 success, could benefit the offspring. One finds nests, it is 

 true, which seem to bear traces of imperfect workmanship, 

 but they have generally followed the destruction of a first 

 attempt and are built therefore hurriedly to receive the eggs 

 already overdue ; yet even they, unfinished though they may 

 be in workmanship, meet the biological end of nest building. 

 I do not deny the existence of nests ill adapted to their 

 surroundings. A change of environment is all that is required 

 to supply the conditions necessary for the appearance of 

 individual variations in constructive ability, some of which 

 will disappear, while others more in harmony with their 

 surroundings will remain ; and I have given my reasons in the 

 history of the Marsh Warbler for believing that such a change 

 of environment is actually in progress at the present time in 

 the case of that particular species. There, as a reference to 

 the illustrations will show, we have different types of nests 

 which seem to bear the stamp of varying degrees of con- 

 structive skill, and all will perhaps in the future become 

 modified before the evolution is complete, since they appear 

 to be either too flimsy or too robust. 



Others regard the passing on of experience by the elder to 



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