ARGENTINE COW-BIRD 77 
considered in relation to the destructive egg-breaking 
habit of the bird, gives it the best chance of being 
preserved; for though the Cow-bird never distin- 
guishes its own egg, of which indeed it destroys a 
great many, a larger proportion escape in a nest 
where many eggs are indiscriminately broken. 
2. The vitality or tenacity of life appears greater 
in the embryo Cow-bird than in other species; this 
circumstance also, in relation to the egg-breaking 
habit and to the habit of laying many eggs in a nest, 
gives it a further advantage. I have examined nests 
of the Scissor-tail, containing many eggs, after incu- 
bation had begun, and have been surprised at finding 
those of the Scissor-tail addled, even when placed 
most advantageously in the nest for receiving heat 
from the parent bird, while those of the Cow-bird 
contained living embryos, even when under all the 
other eggs, and, as frequently happens, glued immov- 
ably to the nest by the matter from broken eggs spilt 
over them. 
The following instance of extraordinary vitality in 
an embryo Molothrus seems to show incidentally that 
in some species protective habits, which will act as 
a check on the parasitical instinct, may be in the 
course of formation. 
Though birds do not, as a rule, seem able to 
distinguish parasitical eggs from their own, however 
different in size and colour they may be, they often 
do seem to know that eggs dropped in their nest 
before they themselves have begun to lay ought not 
to be there; and the nest, even after its completion, 
