THE VEGETABLE WORLD. 287 



tribution, and so passed into the great geographical 

 variety of the vegetable world. The gradual conversion 

 of the universal tropical climate into the present cli- 

 matal zones, may be shown in another very interesting 

 manner, in quite a special instance. All ligneous 

 trunks of Coniferous trees continually increase in thickness 

 at all parts of their circumference. In the equatorial regions, 

 where the climate retains the same character uninterruptedly 

 throughout the year, this thickening of the trunk proceeds 

 without interruption and homogeneously; no mark betrays, 

 in a smooth transverse section of the stem, the time which 

 was required for its formation. As we proceed towards the 

 north, however, as the climatal conditions produce con- 

 tinually increasing diversity in the particular seasons, the 

 corresponding growth in thickness shows itself to have been 

 furthered by the favourable season, and restrained or 

 altogether interrupted by the unpropitious times. In a 

 cross section of a stem are seen, the higher the latitude in 

 which it has grown, the greater differences in the structure 

 of the successive portions of the wood ; until, finally, in the 

 latitudes where there is a severe alternation of winter and 

 summer, so striking becomes the difference between the 

 wood last formed in summer and that first produced in the 

 next spring, that we may count in the number of annular 

 marks thus produced, in a cross section, with great cer- 

 tainty and accuracy, the number of years which have been 

 occupied in the formation of the trunk. The circular lines 

 upon the cross section, well-known to every forester, are 

 thence called the annual rings. When, fortified with the 

 knowledge of this fact, we compare with each other, the 

 trunks of the Conifers which we obtain from the various 

 epochs of formation, we find that the oldest remains exhibit i 

 no trace whatever of annual rings ; but in the course of time 



