laying in the nests of several different species, and of changing the
colour of her eggs according to that of the eggs of the bird in whose
nest she lays. I have inquired from Mr. R. Bowdler Sharpe of the
ornithological department at the British Museum, who kindly gives it
me as his opinion that though cuckoos do imitate the eggs of the
species on whom they foist their young ones, yet one cuckoo will
probably lay in the nests of one species also, and will stick to that
species for life. If so, the same race of cuckoos may impose upon
the same species for generations together. The instinct will even
thus remain a very wonderful one, but it is not at all inconsistent
with the theory put forward by Professor Hering and myself.


Returning to the idea of psychical mechanism, he admits that "it is
itself so obscure that we can hardly form any idea concerning it,"
{139a} and then goes on to claim for it that it explains a great many
other things. This must have been the passage which Mr. Sully had in
view when he very justly wrote that Von Hartmann "dogmatically closes
the field of physical inquiry, and takes refuge in a phantom which
explains everything, simply because it is itself incapable of
explanation."


According to Von Hartmann {139b} the unpractised animal manifests its
instinct as perfectly as the practised. This is not the case. The
young animal exhibits marvellous proficiency, but it gains by
experience. I have watched sparrows, which I can hardly doubt to be
young ones, spend a whole month in trying to build their nest, and
give it up in the end as hopeless. I have watched three such cases
this spring in a tree not twenty feet from my own window and on a