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APPENDIX E 
sterns blot out their foreshortened bodies against the sky.” He 
illustrates what he means by pictures, and states that 44 in the 
night the illusion must often be complete, and most beneficent to 
the hunted beast,” and that what he calls 44 these rear-end sky- 
pictures are worn by most fleet ruminants of the open land, and 
by many rodents with more or less corresponding habits, notably 
hares ” and smaller things whose enemies are beasts of low stature, 
like weasels, minks, snakes, and foxes ; 44 in short, that they are 
worn by animals that are habitually or most commonly looked up 
at by their enemies.” Mr. Thayer gives several pictures of the 
prongbuck and of the northern rabbit to illustrate his theory, 
and actually treats the extraordinarily conspicuous white rump 
patch of the prongbuck as an 44 obliterative ” marking. In reality, 
so far from hiding the animal, the white rump is at night often 
the only cause of the animal's being seen at all. Under one picture 
of the prongbuck Mr. Thayer says that it is commonly seen with 
the white rump against the sky-line by all its terrestrial enemies, 
such as wolves and cougars. Of course, as a matter of fact, 
when seen against the sky-line, the rest of the prongbuck’s 
silhouette is so distinct that the white rump mark has not the 
slightest obliterative value of any kind. I can testify personally 
as to this, for I have seen prongbuck against the sky-line hundreds 
of times by daylight, and at least a score of times by night. The 
only occasion it could ever have such obliterative value would be 
at the precise moment when it happened to be standing stern-on 
in such a position that the rump was above the sky-line and all 
the rest of the body below it. Ten steps farther back, or ten 
steps farther forward, would in each case make it visible instantly 
to the dullest-sighted wolf or cougar that ever killed game; so 
that Mr. Thayer’s theory is of value only on the supposition that 
both the prongbuck and its enemy happen to be so placed that the 
enemy never glances in its direction save at just the one particular 
moment when, by a combination of circumstances which might not 
occur once in a million times, the prongbuck happens to be helped 
by the obliterative quality of the white rump mark. Now, in the 
first place, the chance of the benefit happening to any individual 
prongbuck is so inconceivably small that it can be neglected, and, 
in the next place, in reality the white rump mark is exceedingly 
conspicuous under all ordinary circumstances, and, for once that it 
might help the animal to elude attention, must attract attention 
to it at least a thousand times. At night, in the darkness, as 
anyone who has ever spent much time hunting them knows, the 
white rump mark of the antelope is almost always the first thing 
about them that is seen, and is very often the only thing that is 
ever seen ; and at night it does not fade into the sky, even if the 
animal is on the sky-line. So far as beasts of prey are guided by 
