400 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 
Whether the temperature in question is the mean of a certain period 
or the sum of the daily temperatures for that period, or the sum in 
excess of a certain minimum, expressed in degrees of the thermometric 
seale or in calories, and how to determine the precise beginning and 
ending of this period for each locality, are questions respecting which 
difference of opinion prevails; and authors are not agreed as to whether 
the temperature should be taken in the sunshine or in the shade, or ata 
certain distance below the surface of the earth. At the same time it has 
been demonstrated by Linsser and others that a definite quantity of 
heat is required to complete the process of reproduction in a number of 
plants experimented upon,—and nature’s laws are not framed for isolated 
eases. This law is taken advantage of by expert gardeners and horti- 
culturists who are able to so regulate the temperature of their green- 
houses that they can produce a perfect flower or aripe fruit on a speci- 
fied day. 
A few species (particularly among plants) are so sensitive to cold 
that they are limited in northward range by the line of killing frost- 
but in the vast majority of cases the winter temperature is of no conse- 
quence. As I have already shown, ‘The season of reproduction for 
the plant, as for the animal, is the warm part of the year. After the 
period of reproduction the plant withers; after it flowers and fruits 
and matures its seed, it dies down or becomes physiologically inactive. 
And what the plant accomplishes in one way the animal accomplishes 
in another. To escape the cold of winter and its consequences, the 
sensitive mammal hibernates; the bird migrates to a more southern 
latitude; the reptile and batrachian dig holes in the mud or sand and 
remain in a torpid condition; the insect sleeps in its cocoon or buries 
itself under leaves or decomposing vegetation; and none but the har- 
dier forms of life are left to be affected by winter temperatures.” (NV. 
Am. Fauna, No. 3, September, 1890, 26, 27.) 
After temperature and humidity, several subordinate though impor- 
tant factors remain to be considered. Among these may be mentioned 
the duration and actinic effects of sunlight (governed in part by per- 
centage of cloudiness or fog and by the mechanical purity of the atmos- 
phere). The character of the soil also determines the presence or 
absence of many species.* 
EFFECTS OF HUMIDITY CONTRASTED WITH EFFECTS OF TEMPERA- 
TURE. 
With a few exceptions the Boreal zones, owing to their low tempera- 
tures, precipitate sufficient moisture to support arboreal vegetation, 
and do not possess arid areas. The Transition and Sonoran zones, on 
the other hand, naturally fall into two important subdivisions, arid and 
*The controlling causes of distribution will not be discussed further here because 
they are the subject of another communication upon which the writer is engaged. 
