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ee he a IRSE ea a MINE TURA 
1894.] The Numerical Intensity of Faunas. 755 
the biogenetic factor keeps up or even becomes intensified, the 
species begins its downward course, since numerical diminu- 
tion means final extinction. The biogenetic factor, the influ- 
ence of propagation, will, in all probability, decline with any 
changes in external conditions which affect the physical well- 
being of the organism, so that the sum of influences springing 
from external circumstances and internal conditions work 
conjointly to exhilerate or depress the life of the animal. 
Furthermore, although a new species responds more fittingly 
to its environment and possesses peculiar advantages over its 
companions, this species, it may be assumed, survives because 
it is less at odds with its surroundiugs, not because it is most 
appropriately placed. As it becomes more and more part of 
the new status which brought it into existence, its organism 
more and more nearly attains its limital fecundity. 
The list of possible combinations of conditions upon the 
emergence of a species would then be four. 
First—Favorable Environment and High Vitality—pro- 
creative activity. 
Second.—Unfavorable Environment and High Vitality. 
Third.—Favorable Environment and Low Vitality. 
Fourth.—Unfavorable Environment and Low Vitality. 
The discussion of these four as limital expressions, covers the 
varying phases under which a species attains its numerical 
maximum. And this discussion assumes, for the sake of reach- 
ing definite results, that the species is considered as restrained 
by the boundaries of a limited area, an assumption not very 
much at variance with facts. 
Favorable Environment and High Vitality—In this case the 
species would rapidly rise to its numerical maximum, and 
maintain it as long as the environment and its own vitality 
remained propitious. But this very intensity of development 
would lead to the deterioration of the species, and bring about 
its own extinction. The competition between its own repre- 
_ sentatives would become exasperated through their great 
number, and this would drain the food-supply, while the 
excessive productivity would reduce procreative power. The 
zoological consequence, in this instance, would be quick 
