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VARIABILITY OF SPECIES IN A STATE OF NATURE 71 
size. I noted particularly that these variations bore no 
necessary relation to each other, so that a large temporal 
muscle and zygomatic aperture might exist either with a 
large or a small cranium ; and thus was explained the curious 
difference between the single-crested and the double-crested 
skulls, which had been supposed to characterise distinct species. 
As an instance of the amount of variation in the skulls of 
fully adult male orangs, I found the width between the orbits 
externally to be only 4 inches in one specimen and fully 
5 inches in another. 
Exact measurements of large series of comparable skulls of 
the mammalia are not easily found, but from those available 
I have prepared three diagrams (Figs. 14, 15, and 16), in order 
to exhibit the facts of variation in this very important organ. 
The first shows the variation in ten specimens of the c mnnon 
wolf (Canis lupus) from one district in North America, and 
we see that it is not only large in amount, but that each 
part exhibits a considerable independent variability. 1 
In Diagram 15 we have the variations of eight skulls of 
the Indian Honey-bear (Ursus labiatus), as tabulated by the 
late Dr. J. E. Gray of the British Museum. For such a 
small number of specimens the amount of variation is very 
large — from one-eighth to one-fifth of the mean size, — while 
there are an extraordinary number of instances of inde¬ 
pendent variability. In Diagram 16 we have the length and 
width of twelve skulls of adult males of the Indian wild boar 
(Sus cristatus), also given by Dr. Gray, exhibiting in both sets 
of measurements a variation of more than one-sixth, combined 
with a very considerable amount of independent variability. 2 3 
The few facts now given, as to variations of the internal 
parts of animals, might be multiplied indefinitely by a search 
through the voluminous writings of comparative anatomists. 
But the evidence already adduced, taken in conjunction with 
the much fuller evidence of variation in all external organs, 
leads us to the conclusion that wherever variations are looked 
for among a considerable number of individuals of the more 
1 J. A. Allen, on Geographical Variation among North American Mammals, 
Bull. U. S. Geol. and Geog. Survey, vol. ii. p. 314 (1876). 
3 Proc. Zool. Soc. Land., 1864, p. 700, and 186S, p. 28. 
