322 THE WONDER OF LIFE 



be ' blending ' and ' overlap ' of instincts, and many cases 

 of individual disturbances are on record. A bird may 

 build a new nest at the end of the breeding season ; or 

 it may build a supernumerary nest ; or it may stop nesting 

 and drop the eggs on the groimd ; or it may migrate too 

 soon, leaving its young to perish. The most common 

 failure is in the adjustment of nest-buUding to the time 

 of egg-laying, and at this point ' parasitism ' arose. The 

 lack of attunement between egg-laying and nest-building 

 is casual in many birds, but it became more than casual 

 in cuckoos and cow-birds. A modification of instincts 

 ensued and a modus vivendi was arrived at. As to what 

 started the lack of attunement, we can only say — ' a 

 nervous variation', such as all highly strung creatures 

 frequently exhibit. 



Jenner pointed out that the bird has but a short time 

 to stay in its breeding area, and much to do in that short 

 time. The gain of leaving the eggs to a succession of 

 other birds is manifest. Other naturalists have indicated 

 various advantages which make it easier to understand the 

 development of the habit by selection, but leave the origin 

 of the habit obscure, except that it is well known of many 

 birds that they casually lay an egg in another bird's nest. 



Many years ago we suggested, Uke Prof. Eimer, that the 

 pecuhar habit should be considered as an outcrop of a 

 very peculiar constitution and character. That is to say, 

 the non-brooding is not to be held apart from many other 

 pecuUarities of the cuckoo with which it is congruent. We 

 referred, for instance, to the absence of any ' married 

 life ', to the preponderance of males, to the polyandry that 

 obtains, to the insatiable and gluttonous appetite, as well 

 as to the sluggish interrupted egg-laying. That any 



