bs is seven,
but in Hylobates there are sometimes eight pairs. In Semnopithecus and
Colobus there are generally seven, but sometimes eight pairs of true
ribs. In the Cebidae there are generally seven or eight pairs, but in
Ateles sometimes nine" (_Proc. Zool. Soc._, 1865, p. 568). In the same
paper it is stated that the number of dorsal vertebrae in man is
normally twelve, very rarely thirteen. In the Chimpanzee there are
normally thirteen dorsal vertebrae, but occasionally there are fourteen
or only twelve.

_Variations in the Skull._

[Illustration: FIG. 14.--Variation of Skull of Wolf. 10 specimens.]

Among the nine adult male Orang-utans, collected by myself in Borneo,
the skulls differed remarkably in size and proportions. The orbits
varied in width and height, the cranial ridge was either single or
double, either much or little developed, and the zygomatic aperture
varied considerably in 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 common 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.[23]

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
independent 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, e