244 CAUSES WHICH LIMIT 



obstacle. We can only mark on the map the point where_, the usual 

 law ceasing to apply, one of these two causes begins to act. 



While the effect of temperature on species was sought for only in 

 thermometric averages and the minima of winter, it was impossible to 

 explain why a great number of the species stop precisely in those 

 parts of Europe where the mean temperatures differ least at great dis- 

 tances. Of this, Scotland is the most striking example. A multitude 

 of species have their limit near Edinburgh; to such an extent indeed 

 that the flora of the country beyond the Grampian mountains has 

 always been considered rather an appendage of the floras of Lapland 

 and the Shetland Isles than of the British. Yet the mean of tempera- 

 ture, compared month by month, differs surprisingly little from one 

 extremity of Scotland to the other. The law above stated enables 

 us to understand these facts. Precisely because of the uniformity and 

 slight variability of mean temperatures in Scotland, there elapses a 

 long interval of time between the day when the temperature of 4°, for 

 example, commences, and that when the temperature of 5° begins. 

 If, then, two species are organized in such a manner as to commence 

 actively vegetating — the one at 4°, the other at 5° — the first will re- 

 ceive for a length of time a heat which is useless to the other, and 

 consequently their limits will diverge considerably. It is not the same 

 under an easterly climate, where the transition from 4° to 5°, 6°, 

 and so on, takes place so rapidly that all species begin to vegetate 

 nearly at the same time. Hence, in the west, the limits are influenced 

 especially by the initial and final temperatures necessary for each 

 species; in the east by the sum total of heat. 



The examples on which I have relied are drawn from plants of the 

 centre and north of Europe. I have no doubt but that in countries 

 analogously situated, whether in Asia or North America, we should 

 find the same facts with regard to other species. It would only re- 

 quire that those regions should be as well known as Europe to enable 

 us to verify in detail both the temperatures and the limits. More to 

 the south dryness and humidity seem the principal causes of the 

 limitation of species. Besides, when the temperature operates it is in 

 those regions much more uniform and much more proportional from 

 one season to another, in all localities similarly situated, which is the 

 reason why the average of the year or the season is competent to re- 

 place the complicated law which governs species. Indeed, on the 

 borders of the Mediterranean sea, tlie limits have appeared to me so 

 often determined by the humidity, or by causes still unknown, that 

 the operation of the temperature has almost always eluded my calcu- 

 lations. 



The law with which I have been occupied has its application no 

 doubt to the limitation of species as regards altitude. It will show 

 why it is that species do not observe the same relative distances on 

 the flanks of different chains of mountains ; why, in other words^ the 

 limits of height cross one another in the same way with the limits on 

 the surface of a continent. 



It is also probable that by means of this law we shall be able to explain 

 the periods of flowering and maturing for species in different localities 



