436 The American Naturalist. [May, 
“ What increases the difficulty of the problem is, for example, the fact 
that besides the conforming power of many external factors, and besides 
the adaption of species to these factors, there are innate, hereditary dis- 
positions, that from inner unknown causes produce structures which 
we cannot bring into any relation to the surrounding natural condi- 
tions, at all events not to those present, and which we cannot therefore 
understand. These inner dispositions, different according to the natural 
relationship, bring along with them this, that the development of the 
species under the influence of the same factors can lead to the same 
result in entirely different ways. While, for example, one species adapts 
itself to a dry habitat by means of a thick coating of hair, another 
species under the same conditions cannot bring forth a single hair, but 
choses, for example, to cover itself with a layer of wax, or to reduce its 
foliage and become a stem-succulent, or becomes ephemeral in its de- 
velopment.” This paragraph explains fairly well the scope of the 
problem which the author endeavors to solve. 
The term “ Lebensform ” is further explained. The cactuses, fleshy 
euphorbias and the succulent stapelias, though belonging to widely 
different families all possess the same life-form, 
In a certain area or habitat, certain species have adapted themselves 
to the conditions there present, and so form a plant community (Pflan- 
zenverein). Of course these plant iti tal e sharply 
divided, and the same species may occur in more than one community. 
But, nevertheless, each community possesses its peculiar physiognomy. 
“ Ecological plant geography must deal with the following : | 
“1. The factors of the outside world that play a rôle in the economy of 
plants, and the action of these factors upon the external and internal 
structure of plants, upon the life-period and other ecological relations, 
as well as upon the topographical limits of the species. 
“2, Grouping and characterizing of the classes of communities 
(Vereinklassen) present upon the surface of the earth. 
“3. The conflict between the communities.” 
The first section of the work deals with the ecological factors. The 
atmospheric factors are light, heat, moisture and air currents. The 
terrestrial factors are composition and physical condition of the soil, 
air and water in the soil, and other physical characters ; the effect of 
dead or living mantle (snow or fallen leayes) upon the surface, of ani- 
mals (earth-worms, moles, etc.), or plants (fungi, bacteria) beneath the 
surface of the soil, and finally, the effect of the direction and height of 
mountain chains, angle and direction of slope, and similar considera- 
tions. 
