252 



vegetable world. The gradual conversion of the universal tropical 

 climate into the present climatal 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 re- 

 tains the same character uninterruptedly throughout the year, this 

 thickening of the trunk proceeds without interruption, and homogene- 

 ously ; 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 continually in- 

 creasing 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 an- 

 nular marks thus produced, in a cross section, with great certainty and 

 accuracy, the number of years which have been occupied in the forma- 

 tion 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 no trace whatever 

 of annual rings ; but in the course of time they become continually 

 more defined, so that lastly, in the most recent formations, for instance 

 in the upper brown coal, they appear marked just as distinctly as in 

 the trees now living in the same localities." — p. 286. 



The assumption of repeated creations advocated by some modern 

 naturalists finds no favour with our author, who holds the idea " of a 

 totally new origination of vegetable germs, out of unorganized or even 

 inorganic matters, to be superfluous, and therefore not to be admitted; 1 ' 

 he is content to trace back the multifarious forms of vegetation which 

 now adorn the earth to the acknowledged first and simplest form — 

 the cell, which we know can and does vegetate as an independent 

 plant. And, as shown in the preceding sketch, 



" We see that the vegetable world begins in water, under the simplest 

 forms, and in that very family in which the whole plant is represented 

 by a single cell, most frequently, in the present time. In the succeed- 



