482 LOCALIZATION OF PLANTS. 



gan to assert their influence through the lowering of the surface temper- 

 Change in the ature of the globe. The first of these events was not alone 

 tbTa7mos n f l iimte d in i ts effect to a Disturbance of the organic functions 

 phere. of plants by diminishing the amount of gaseous material from 



which they gathered their support in the air : its influence was also felt 

 in animal life by rendering that possible which was not possible before 

 the existence of the quickly-respiring and hot-blooded tribes ; for it fol- 

 lows as a chemical necessity that, under the circumstances of the case, 

 the removal of the carbonic acid was attended with the evolution of an 

 equal volume of oxygen gas. As respects the influence of the sun, which 

 gradually led to the establishment of climates, first in an order of time, 

 and then in an order of place, this was the signal for the localization of 

 Definite locaii- P^ ants an( l animals i* 1 definite regions. From many coun- 

 zation of plants tries which they had thus far inhabited they were now ex- 

 and animals, p^^ an( j "barriers of temperature placed around them which 

 they could never again overpass. And as these great changes occurred, 

 they were attended by the extinction of countless forms in both king- 

 doms, which were utterly unable to maintain themselves in the new cir- 

 cumstances around them, their places being occupied by the extension 

 of contemporaneous forms, or by the appearance of others that were whol- 

 ly new. 



As an illustration of the manner in which a vegetable organism may 



Exam le of the ^ e use( ^ * n ^^ s i nverse Wa 7 f r tne determination of physic- 

 inverse method al conditions, I may introduce the following quotation from 

 len * Schleiden : " The gradual conversion of the universal trop- 

 ical 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 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 toward the north, however, 

 as the climatal conditions produce continually-increasing diversity in the 

 particular seasons, the corresponding growth in thickness shows itself to 

 have been furthered by the favorable 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 differ- 

 ences in the structure of the successive portions of the wood, until final- 

 ly, in the latitudes where there is a severe alternation of winter and sum- 

 mer, 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 



