474 INFLUENCE OF PHYSICAL CONDITIONS. 



all the physical conditions were identical, yet, since heat is only one of 

 these conditions, it alone is insufficient, and that differences in the press- 

 ure of the air, the amount of moisture, the quantity of carbonic acid, as 

 also variations in the constitution of the soil, must have their effect. In- 

 stead, then, of limiting our views to the control of temperature over the 

 occurrence of plants, we must enlarge them in such a way as to include 

 divers other influences, some of which are those just mentioned, and all 

 are equally of a physical kind. 



This therefore brings before us, in an impressive manner, the subject to 

 which this chapter is devoted, the influence of physical agents generally 

 over organization. 



That the conditions of temperature alone are insufficient to account for 

 the occurrence and distribution of plants may be clearly established by 

 the aid of another series of facts. Throughout the old continent, with 

 the exception of its torrid zone, from the south of Africa to the north of 

 Europe, heaths abound, their species being very numerous in the south- 

 ern latitudes, less so in the northern, but the individuals increasing in 

 number as the species diminish. At the extreme north the common 

 heather remains as the sole representative of the whole group, and so 

 universally covers the surface as to give a characteristic feature to the 

 landscape. But in America, which reaches through all corresponding 

 degrees of latitude, and has in its proper localities the same mean tem- 

 peratures, not a single heath ever occurs. Again, in the New World, 

 through forty degrees on each side of the equator, the cactus tribe of all 

 kinds of grotesque forms abounds, but in Africa, though there are local- 

 ities of corresponding temperature, not a single cactus is to be seen. The 

 spurges there make their appearance. So, again, in Australia, the forests 

 present a melancholy and shadeless character from their leafless casuari 4 - 

 nas, acacias, and eucalypti, whereas, if temperature alone were concern- 

 ed, they should offer the same aspect as the forests of North America 

 and Europe. 



Restricting our examination for the present to the influence of heat, it 

 Influence of may be observed that this is by no means so simple as might 

 h^atTand win- at ^ rst a PP ear - I* s distribution does not correspond with 

 ter colds. the latitude, the lines of equal mean temperature, isothermal 

 lines, not coinciding with the parallels of latitude. If we examine the 

 zones of plant distribution just described, we find that they follow the 

 isothermal lines much more closely than the latitudes ; but even here, 

 again, there are very great deviations deviations which, however, are to 

 some extent understood when we recall that it is not so much with the 

 mean annual temperature that plants are concerned as with the special 

 temperature of particular moments of the year. For the most part they 

 are affected by the heat of the summer season, which is their period of 



