. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Chromatophores and Color Change 715 er^^^ has pointed out the possibiHty of an alternative interpretation, that the pigment cells in these forms may show direct responses when their state is not determined by a dominating mechanism. In summary we appear to find among the reptiles, as with fishes, varying degrees of evolution of chromatophore control from systems involving a probably primitive hormonal mechanism of coordination of the chromato- phores, through those in which both nerves and hormones cooperate, to
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Chromatophores and Color Change 715 er^^^ has pointed out the possibiHty of an alternative interpretation, that the pigment cells in these forms may show direct responses when their state is not determined by a dominating mechanism. In summary we appear to find among the reptiles, as with fishes, varying degrees of evolution of chromatophore control from systems involving a probably primitive hormonal mechanism of coordination of the chromato- phores, through those in which both nerves and hormones cooperate, to those largely dominated by direct nervous innervation. FUNCTIONAL SIGNIFICANCE OF CHROMATOPHORES AND COLOR CHANGE Since the responses of the chromatophores are predominantly responses to color or shade of background, one is led to the hypothesis that the color changes contribute significantly to the obliterative coloration of the ani- mal for protection or aggression and hence increase its chances for sur- vival. One of the few experimental demonstrations that physiological color. •Species of Fishes Fig. 275 ninutes'after c/ionfe of backgrvund Fig. 276 Fig. 275. Percentage choice of a black over a white background for each of eight species of fishes adapted to black and white backgrounds. From Brown and Thompson.'*^ Fig. 276. The rate of change in percentage choice of a black background following transfer of Ericymha from a black to a white background and vice versa. From Brown and Thompson.^ changes do actually increase chances for survival, a view often questioned, is that of Sumner,^*^^ in which he found that fishes given time to change their coloration were seized in smaller percentages by a predatory bird than were unadapted ones. It has also been clearly demonstrated that fishes which are black-adapted tend to select black backgrounds, when given a choice of black or white, more commonly than do white-adapted individu- no {^pjg 275), and that the rate of change
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