Prof. J. Van der Hoeven on Animal Organization. 209 
The foregoing dimensions will show the different sizes of the 
teeth and bones. And as the length of the tibia (minus the 
central process in the knee-joint) in general averages one-fourth 
of the height of the horse to which it belongs, in this way a fair 
approximation to the sizes of the animals under consideration 
may be obtained. It will be observed, taking the measure- 
ments of the crania from the anterior edge of the superior 
maxillary to the orbital cavity, that this part in the fossil 1s 
$ inch longer than that of the pony, and 24 inches shorter than 
the same part in the cart-horse. As this part of the horse’s skull 
is on an average about | inch longer than the space from the an- 
terior margin of the orbital cavity to over the occipital condyles, it 
will be apparent that, if we make a proportional allowance for 
this part, absent in the fossil, and of course add for the fore part 
of the intermaxillary bone, we shall not be far wrong in esti- 
mating the fossil skull at 2 inches longer than the pony’s, and 
about 5 inches shorter than the skull of the cart-horse ; while it 
will be seen that, taking the total of the antero-posterior dia- 
meter of the molars, irrespective of details, these organs in the 
fossil exceed those of the pony by 1 inch, and are only } inch less 
than those of the cart-horse. 
XXV.—Some Remarks on the Succession and Development of 
Animal Organization on the Surface of our Globe, in the dif- 
ferent Periods of its -Hxistence. By J. Van per HorEven, 
Professor of Zoology, University of Leyden *. 
Ir requires but little knowledge of organized bodies to remark 
that there is a great difference in their structure, and that some 
are more, others less complicated. This greater development 
depends not only on the presence of parts or organs which are 
absent in more simple organisms, but also on modifications in 
the structure of parts which exist as well im more simple as 
in more perfect species. In the animal kingdom, for instance, 
there are species which are devoid of the organs of the senses of 
sight and of hearing, so important in man; others which have 
these organs, but in a very different degree of complication. 
Thus the organ of hearing presents a greater number of distinct 
parts in mammals than in fishes; and thus, too, the eye is in 
general more complicated, more moveable, more nicely protected 
in the former than in the latter. It is needless to give a larger 
number of examples of this diversity of perfection. From the 
observation of this diversity originated a conception which seems 
* Written in Dutch, in 1858, before the publication of Mr, Darwin’s 
work. Communicated by Dr. J. Barnard Davis, I’.S.A. 
Ann. & Mag, N, Hist, Ser.3, Vol. xiv. 14 
