CUCKOO PUZZLES 43 
been envolved in adaptation to the risks of the para- 
sitic habit as to have been the cause of it. Very 
significant, however, is the interrupted egg-laying, 
for it appears that the mother lays five to seven 
on alternate days, and then stops, resuming after a 
short interval with a second lot of four or five. 
This would not fit in well with personal incubation, 
but it is congruent with the parasitic habit. Again, 
since the adult cuckoos feed very largely on hairy 
caterpillars, which become scarce after midsummer, 
there is an economic reason for the early migration 
and for leaving the care of the young to others. (3) 
Probably, however, we get most light on the prob- 
lem when we adopt Professor F. H. Herrick’s sug- 
gestion that the loss of the nesting instinct is due 
to an irregularity in the rhythm of the life-cycle 
—a formula which covers many a variation among 
animals. A constitutional change of deep germinal 
origin leads to the suppression of one chapter and 
the lengthening out of another, and just as one 
kind of bird may take to building supernumerary 
nests, another may take to skipping nest-building 
altogether. A lack of attunement between egg- 
laying and nest-making is casual in many birds; 
it has become established in cuckoos because it 
was congruent with some other peculiarities of 
constitution and habit, and because it was found to 
work well.* 
Another great puzzle concerns the cuckoo’s eggs. 
*See J. Arthur Thomson, The Wonder of Life (Melrose, 
London, 1914), p. 315. 
