The Principles of Palaeontology. 15 Y 



§ 3. Effects of External Causes. 



Adaptation.— The adoption of the hypothesis of the influence 

 of the medium furnishes an immediate explanation of the 

 innumerable cases of adaptation which are observable in the two 

 organic kingdoms. Natural selection alone would, moreover, 

 furnish, in many cases, a suflacient interpretation of the phenom- 

 ena observed. 



Adaptatio7i is the fact that types which, in the sum of their 

 characteristics, manifestly belong to the same group, present dif- 

 ferences which are in direct relation to their especial mode of 

 life. Thus the Cheiroptera differ from all the Mammals by their 

 adaptation to aerial locomotion; the Pulmonates are the only 

 Mollusks (with three or four exceptions) adapted to respiration 

 in the air ; the limbs of the Cetacea permit only aquatic locomo- 

 tion, etc. 



. The phenomena of adaptation have been particularly eluci- 

 dated by Greoffroy Saint-Hilaire, who demonstrated that in the 

 same group the organs adapted to diverse functions are referable 

 to one and the same type. He established, for example, the 

 homology of the parts of the skeleton of the vertebrates, what- 

 ever the functions to which they are applied in the diverse 

 forms. From this it is but one step to a reasonable explanation 

 of those diversities by the hypothesis that modifications are act- 

 ually and gradually produced at the expense of the primitive 

 type. This step Geoffrey unhesitatingly took. 



We must include in this order of phenomena all cases of 

 mimicry, premonitory coloring, etc., on which Darwin and 

 Wallace so urgently insisted. 



Zoology and Botany display at every step examples of this 

 important phenomenon. Palaeontology places it within our 

 power to grasp this phenomenon of function, and in many cases 

 shows how the gradual transformations are produced. 



The most celebrated example is that drawn from the study of 

 the fossil forms which are considered as representing the series 

 of progenitors of the horse. It is well known that among those 

 animals the cubitus and the radius are rudimentary, that each 

 limb presents but a single finger, by the sides of which are two 

 small stylets, which represent, in the rudimentary state, the 

 fingers 2 and 4 of the other Mammals. These fingers are very 

 much elongated. 



Put there has been found in Europe, and especially in 

 America, at a period later than the Lower Eocene, an entire 



