158 Report of the State Geologist. 



series of types in which these characteristics of adaptation are 

 more and more strongly . marked. The first of these types^ 

 Eohippus, has one cubitus, and one fibula very distinct, four 

 fingers, and one rudiment on the fore foot, three on the hind 

 foot. This animal, small of size, presents in but a slight degree 

 the differential characteristics of the horse, but through all the 

 succeeding generations the characteristics in question make their 

 appearance little by little, by a very moderate gradation 

 (Marsh, Huxley)* 



The adaptation to flight of the anterior limbs of birds is 

 brought aoout by a process of evolution, several terms of which 

 are known to us. Among ordinary birds the fingers of the 

 anterior extremity are shortened in such a manner that one of 

 the fingers is only a weak stump ; the remainder of the hand 

 is reduced to three metacarpals united and bearing one or two 

 phalanges. The extremity of the wing can only execute move- 

 ments of flexion of small scope. But the most ancient bird 

 known, the ArchcBopteryx of the Upper Jurassic presents a much 

 less degree of regression ; three fingers are well represented and 

 separated ; the middle finger has three phalanges, the others two, 

 and the fingers terminate by claws so that the hand is adapted 

 to prehension. The embryos of the ostrich possess character- 

 istics between these two extremes. Other details relating to the 

 power of flight are also seen in other organs, and to a less 

 degree in Archceopteryx which is closely related to the reptiles, 

 than in the living birds. In the course of this work we shall 

 refer to numerous cases of the same kind. 



The organs most apt to undergo modifications are naturally 

 those which serve the animal in its relations with the external 

 world ; such especially are the members which are employed in 

 prehension, progression, flight, leaping, swimming, and in the teeth 

 which are adapted to the food of the animal. 



But in many instances the entire form of the animal may be 

 modified by causes of the same kind, and Palaeontology some- 

 times leads to the discovery of the gradual progress of this 

 evolution. 



Correlation. — Generally speaking, the adaptation of any type 

 to a determinate mode of existence is not confined to the modifi- 

 cation of a single organ ; for example, the transformation of a 

 terrestrial vertebrate into an aerial type supposes simultaneous 

 modifications in various parts of the skeleton, in the muscular 

 system of the limbs, and also in other parts of the organism. 

 Thus among adult birds the anterior and posterior extremities 



* Marsh, Lecture on the Introduction and Succession of Vertebrate Life in America. {Nature, vol. 

 XVI, p. 471.) 



