86 BIRDS OF LA PLATA 
and of the Spine-tail, Synallaxis hudsoni. Yet these 
birds are beautifully adapted in structure, coloration, 
and habits to their station. It thus happens that in 
districts where the Molothrus is abundant, their eggs 
are found in a majority of the Pipits’ nests; and 
yet to find a young Cow-bird out of the nest is a 
rare thing here, for as soon as the young birds are 
able to quit the nest and expose themselves they are 
mostly carried off by the Hawk. 
CONJECTURES AS TO THE ORIGIN OF THE PARASITIC 
Instinct IN M. bonariensis 
Darwin’s opinion that the “ immediate and final 
cause of the Cuckoo’s instinct is that she lays her 
eggs not daily, but at intervals of two or three days ”’ 
(Origin of Species) carries no great appearance of 
probability with it ; for might it not just as reasonably 
be said that the parasitic instinct is the immediate 
and final cause of her laying her eggs at long inter- 
vals ¢ If it is favourable to a species with the instinct 
of the Cuckoo (and it probably is favourable) to lay 
eggs at longer intervals than other species, then 
natural selection would avail itself of every modifi- 
cation in the reproductive organs that tended to 
produce such a result, and make the improved 
structure permanent. It is said (Origin of Species, 
chapter vii.) that the American Cuckoo lays also at 
long intervals, and has eggs and young at the same 
time in its nest, a circumstance manifestly disadvan- 
