46 Merriam Geographic Distribution of Life. 



period or the sum of the daily temperatures for that period, or 

 the sum in excess of a certain minimum, expressed in decrees 

 of the thermometric scale 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 sun or in the shade, or at a certain distance be- 

 low the surface of the earth. At the same time it ha^ 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 cases. This law is taken advantage of by 

 expert gardeners and horticulturists who are able to so regulate 

 the temperature of their green-houses that they can produce a 

 perfect flower or a ripe fruit on a specified 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 tem- 

 perature is of no consequence. 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 vege- 

 tation ; and none but the hardier forms of life are left to be 

 affected by winter temperatures." (N. Am. Fauna, No. 3, Sep- 

 tember, 1890, 26-27.) 



After temperature and humidity, several subordinate though 

 important factors remain to be considered. Among these may 

 be mentioned the duration and actinic effects of sunlight (gov- 

 erned in part by percentage of cloudiness or fog and by the me- 

 chanical purity of the atmosphere)- The character of the soil 

 also determines the presence or absence of many species.* 



* 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. 



