INTRODUCTION. 51 



ed with life attains completeness as we see the species, genera, 

 and entire families belonging to one hemisphere, reflected, as 

 it were, in analogous animal and vegetable forms in the oppo- 

 site liemisphere. These are, so to speak, the equivalents w^hich 

 mutually personate and replace one another in the great series 

 of organisms. These connecting links and stages of transition 

 may be traced, alternately, in a deficiency or an excess of de- 

 velopment of certain parts, in the mode of junction of distinct 

 organs, in the differences in the balance of forces, or in a re- 

 semblance to intermediate forms which are not permanent, 

 but merely characteristic of certain phases of normal devel- 

 opment. Passing from the consideration of beings endowed 

 with life to that of inorganic bodies, we find many striking 

 illustrations of the high state of advancement to which modern 

 geology has attained. We thus see, according to the grand 

 views of Elie de Beaumont, how chains of mountains dividing 

 different climates and floras and different races of men, reveal 

 to us their relative age, both by the character of the sediment- 

 ary strata they have uplifted, and by the directions which 

 they follow over the long fissures with which the earth's crust 

 is furrowed. Relations of superposition of trachyte and of 

 syenitic porphyry, of diorite and of serpentine, which remuin 

 doubtful when considered in the auriferous soil of Hungary, 

 in the rich platinum districts of the Oural, and on the south- 

 western declivity of the Siberian Altai, are elucidated by the 

 observations that have been made on the plateaux of Mexico 

 and Antioquia, and in the unhealthy ravines of Choco. The 

 most important facts on which the physical history of the 

 world has been based in modern times, have not been accu- 

 mulated by chance. It has at length been fully acknowledg- 

 ed, and the conviction is characteristic of the age, that the 

 narratives of distant travels, too long occupied in the mere 

 recital of hazardous adventures, can only be made a source of 

 instruction where the traveler is acquainted with the condi- 

 tion of the science he would enlarge, and is guided by reason 

 in his researches. 



It is by this tendency to generalization, which is only dan- 

 gerous in its abuse, that a great portion of the physical knowl- 

 edge already acquired may be made the common property of 

 all classes of society ; but, in order to render the instruction 

 imparted by these means commensurate with the importance 

 of the subject, it is desirable to deviate as widely as possible 

 from the imperfect compilations designated, till the close of 

 the eighteenth century, by t^e inappropriate term of popula? 



