238 CAUSES WHICH LIMIT 



basis on which we must always rest. Nobody disputes this with re- 

 gard to the work of classification, and experience proves that the same 

 is the case as respects the study of the geographical distribution of 

 organized beings. 



Having been occupied several years with the subject of botanical 

 geography, I have often had my attention recalled to a fundamental 

 problem, which will serve to explain many others in the same science. 

 This problem is to ascertain in what manner and after what laws spe- 

 cies are arrested in their geographical expansion, and this in the 

 simplest of conditions, on the surface, namely, of a continent apart 

 from the consideration of any mountains which may traverse it. It is 

 easy to conceive that the determination of the boundaries of species must 

 draw after it that of their proportion according to families in each 

 country, and that it connects itself with important questions in physi- 

 ology and agriculture. It is clear, also, that geologists and physicists 

 require to know to what extent the presence of the same species at two 

 epochs or in two countries determines the analogy of climates, and with 

 what degree of precision the geographical limit of a species proves an 

 equality in the exterior conditions of temperature. 



The questions which arise out of this subject have almost always 

 reference to the limit towards the north, or to speak more exactly, to 

 the polar limit or that lying towards one of the poles. I leave out of 

 consideration, therefore, all that relates to the southerly limit. 



On the subject of the polar limits opinions have changed with the pro- 

 gress of physical geography. Originally nothing but the mean annual 

 temperature of climates was observed ; and on comparing the limits of 

 species with this standard singular anomalies presented themselves. 

 In 1815 and 1817 M. de Humboldt introduced into physical geography 

 an important principle, viz : the comparison of lines passing across the 

 points which offer the same mean degree of temperature during the 

 year, the three months of winter, and the three of summer, being the 

 lines we term isothermal, isochimenal, and isotheral. This illustrious 

 scientist taught us that the mean temperature of seasons is of more 

 importance than that of the year, and that, in general, two similar 

 climates may be distributed into fractions very dissimilar, and which 

 neutralize one another in the estimate of the mean annual temperatures. 

 From this we might judge that the temperatures of seasons or monthly 

 temperatures would explain the habitat of species, or in other words, 

 that each species advances over a continent to a certain line which 

 marks an equal temperature during some period of the year, unless it 

 should be arrested by a climate too dry or too humid, or by a material 

 obstacle, such as the sea. I have believed and have heretofore said,* 

 that annual species ought to be limited very nearly according to iso- 

 theral lines, because their vegetation is confined in whole or in great part 

 to the three months of summer. It has seemed to me that the peren- 

 nial or ligneous species ought often to be limited to the lines of equal 

 temperature during some months of the mild season, or by the com- 

 parative shortness of the winters when the question regards plants 



^ Geographical disir^u'ion ff alimeniari/ J lani!. Bibl. Univ^r . de Geneve. Apnl and May, 

 1836. 



