Consequently, as the imparting of heat promoted the process of 
hatching, those individuals which most constantly cuddled or brooded 
over their eggs would, other things being equal, have been most 
successful in rearing progeny ; and so the incubating instinct would 
be developed without there having been any intelligence in the 
matter.”—(Art: Instinct. In Encyclopaedia Britannica. ed., 1888.) 
We are told that the incubating process began by warm 
blooded animals showing that kind of attention to their 
eggs which we find is frequently shown by cold-blooded 
animals, who are only anxious to protect the eggs from 
destruction by enemies, and without any thought of hatch 
ing them out. 
But in reply to this it is not quite certain that cold 
blooded animals are content with simply protecting their 
egg s , for we are told that “ the pythons (at all events when 
in captivity) coil themselves round their eggs and keep up 
a temperature very considerably above that of the sur 
rounding medium.”* But if the python is thus careful 
about maintaining the temperature of the eggs, under 
circumstances which render it necessary, what ground 
have we for supposing that the warm-blooded animals 
will not do the same ? 
In this passage I understand that it is implied that 
all incubation is the result of a blind instinct. But 
certainly this is not so, for, as Büchner says :— 
“ What has instinct to do with it when the ostrich, like many other 
birds, leaves the business of hatching its eggs to the sun during the 
day, and only covers them with its body during the cool of the night ? 
Or when the same bird, acting in this way in Senegal never leaves 
its eggs day or night at the Cape of Good Hope, where the warmth 
of the air is less ? Or when geese and ducks in our moderate 
climate leave their eggs for awhile without any care, while the same 
birds in the polar regions in such a case cover their nests with 
feathers as a protection against the cold?”—(Mind in Animals, 
p. 19.) 
* Chambers' Encyclopaedia. Art: Reptiles.