y not be of very great importance.

In the present state of our knowledge, all attempts at explanation are,
of course, of the nature of hypotheses. If further investigations should
disclose this explanation to be untenable, then we would either have to
suppose some unknown power in the eye of the horse,[AB] or else seek a
cause in the animal's brain. Further experiments on other horses would
be necessary in order to discover whether the species as a whole
possesses this ability or whether only certain ones are thus endowed.
The former is of course more probable. In this particular case
conditions were unusually favorable for the development of this
ability. We must bear in mind that in all probability Mr. von Osten's
movements very gradually became as minute as they are now, and that
therefore Hans at first learned to react to such as were relatively
coarse. Furthermore, his practice extended throughout four years and
during this time it was his sole occupation. Without specific
predisposition, however, all this practice would have been utterly
futile. We can also readily appreciate how indispensable in the struggle
for existence a well-developed power of perceiving moving objects must
be to horses (and most other animals) living in their natural condition
and habitat, in order to be aware of the approach of enemies, or, in the
case of carnivora, the presence of prey. In view of all these
considerations we can readily see how it was possible that the horse,
perhaps in spite of rather defective vision, could react with precision
to movement-stimuli which escaped observation by human eyes.

[Footnote AB: Koenigshoefer, who as we have already said, seconds the
explanation given by the ophthalmologist Berlin (and who confounds
"Butzenscheiben" astigmatism with the common, so-called regular
form), believes[54] that not only astigmatism but also the shape of
the blind-spot of the eye must be taken into consideration. This
portion of the retina, where the fibres of the optic nerve enter the
eye (and called "blind-spot" because there are no cells there that
are sensitive to light) is very nearly circular in man, but differs
in shape in the different species of animals. Koenigshoefer thought he
had discovered that a relatively elongated blind spot was favorable
to keenness of vision. If we place the mammalia in series on the
basis of their relative keenness of vision, he says, we w