ery wonderful indeed. It is not
my purpose to discuss instinctive protective habits in this chapter; I
wish rather to call attention to two _senses_,[93] which are to be
observed in certain of the lower animals, and which man and some of the
higher animals have lost in the process of evolution. I refer to
tinctumutation, the "color-changing" sense, and the sense of direction,
or, as it is commonly and erroneously termed, the "homing instinct."
Neither of these faculties is instinctive, but they are, on the
contrary, true senses, just as hearing, or taste, or smell is a sense.
Careful dissections and repeated experiments have shown me, beyond
peradventure, that these two psychical habitudes have their centres in
the brains (ganglia) of animals which possess them.

[93] I believe that I am the first to claim the _sensual_ importance
of tinctumutation and the sense of direction or the "homing sense."
Heretofore they have been regarded, by all authorities as far as I
know, as instinctive in character.--W.

The chromatic function--and I use this term to designate the faculty of
changing color according to surroundings--is possessed by a number of
the lower animals. The chameleon is the best known of all the
tinctumutants (_tinctus_, color, and _mutare_, to change), though many
other animals possess this faculty in a very marked degree. In order to
understand the manner in which these changes or modifications of color
take place, one must know the anatomy of the skin, in which structure
these phenomena have their origin. The frog is a tinctumutant, and a
microscopic study of its skin will clearly demonstrate the structural
and physiological changes that take place in the act of tinctumutation.
The skin of a frog consists of two distinct layers. The epidermis or
superficial layer is composed of pavement epithelium and cylindrical
cells. The lower layer, or _cutis_, is made up of fibrous tissue,
nerves, blood-vessels, and cavities containing glands and cell elements.
The glands contain coloring matter, and the changes of color in the
frog's skin are due to the distribution of these pigment-cells, and the
power they have of shrinking or contracting under nerve irritation. The
pigment varies in individuals and in different parts of the body. Brown,
black, yellow, green, and red are the colors most frequently observed.
The color-cells are technically known as _chromatophores_. If the web
of a frog's foot be placed on the