438 M. Rerrea on Glyptotlon ornatus. 



roughened with rugosities, with the lozenge-shaped synarthro- 

 dia! surface of the lower margin of the ischium. A small diar- 

 throdial facet also corresponds with that which terminates this 

 margin. 



At the level of its articulation with the ischium, the transverse 

 apophysis bends suddenly downwards nearly at a right angle. 

 It becomes vertical, and continues the outer face of the ischium 

 downwards for some centimetres. I may remark, in conclusion, 

 that this arrangement of the second coccygeal vertebra of the 

 sacrum, which is thus suddenly bent downwards in G. ornatus, 

 occurs also upon all the middle caudal vertebras of the different 

 species of Glyptotlon. 



When we take up the study of fossils we are inevitably led 

 towards the highest questions of zoology. These ancient spoils 

 of an extinct world bear imprints which the genius of Cuvier 

 taught us to read — imprints by the aid of which we can approxi- 

 mate the action of the forces at such remote periods to the 

 action of the forces which are still exerted upon the develop- 

 ment of organized beings. 



In organized beings there exist constant differences of orga- 

 nization, upon which is founded their arrangement in classes, 

 families, genera, and species. From these differences, and from 

 them alone, results the variety in the animal and vegetable 

 kingdoms ; but is the cause or principle of this variety, or of 

 these differences, inherent in the vegetable or animal organism? 

 or is it in great part exterior to it ? 



Let us remark, in the first place, that whatever maybe the oppo- 

 sition of these two methods of looking at the differences of 

 plants and animals, there is one idea and one word which neither 

 of them has been able to eliminate — the idea and the word 

 type. For it is certainly necessary to recognize differences be- 

 tween organisms, diverse imprints, and to give them a name. 

 But while some believe that every specific difference corresponds 

 to an immutable type, others think that there exists only one 

 type indefinitely modifiable by the action of the media in which 

 it is developed. 



This latter hypothesis, to which Mr. Darwin's work on the 

 origin of species (which, at the time of its appearance, produced 

 so deep a sensation among naturalists) referred, appears to me to be 

 equally irreconcileable, on the one hand, with what logic compels 

 us to admit philosophically, and, on the other, with the facts of 

 experience. In fact, this unique type, not being immutably de- 

 termined, would not be so essentially, which destroys the very 

 idea of type, and substitutes in its place that of the indetermi- 

 nate substance. Notwithstanding the development given to this 



