114 A PHILOSOPHER WITH NATURE 



that the nests commonly contain young by the time 

 the last egg is laid. The evidence would seem to 

 suggest that this habit has been developed in the 

 cuckoo rather as the result of its other habits than 

 as the cause of them. 



There is another theory which has received the 

 adherence of many persons of weight. One of the 

 strangest of many unusual facts regarding the cuckoo 

 is the proportion of the sexes. The males greatly 

 outnumber the females. The males have been 

 estimated at ten to each female, and by some 

 observers as high as fifteen to one ; even the most 

 moderate estimates do not place the proportion at 

 less than five to one. The theorists who find in 

 this fact the cause of the peculiar habits of the 

 birds are, however, not agreed among themselves 

 as to how it has operated. Some regard it as pre- 

 cluding the cuckoo from mating in the ordinary 

 way, and so from building a nest and rearing her 

 young. Others regard the temperament of the 

 bird as a kind of physiological accompaniment of 

 the relationship of the sexes, but on grounds which 

 seem rather unsatisfactory, if not obscure. 



It is probable that any satisfactory explanation 

 of the unusual habits of the cuckoo must be sought 

 for in the operation of natural selection. The great 

 difficulty is, however, to find the key of the situation. 

 Why has the cuckoo developed in a certain direction 

 and become such an exception to other birds ? 

 Many of the peculiarities which observers have taken 

 for causes are without doubt effects acquired after 

 the bird had already made progress in a certain 

 direction. But what has been the starting point, 

 and where are we to find the cause which first led 



