Prof. J. Van der Hoeven on Animal Orgctnization. 209 



Tlie 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 obser\ed, 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 is 

 I inch longer than that of the pony, and 2i inches shorter than 

 the same part in the cart-horse. As this part of the horse's skull 

 is on an average about 1 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 Existence. By J. Van deii Hoevkv, 

 Professor of Zoologj', University of Leyden*. 



It 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 moditications in 

 the structure of parts which exist as well in 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 

 ])arts 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 185S, l)cforc the publication of Mr. Darwin's 

 work. Couimunicateit by Dr. J. Bai-nurd Davis, F.S.A. 



Ann. i^- Mag. N, Hist. Scr.3. Fo/.xiv. 14 



