380 
On Climate and the Supply of Labour 
Scottish summer, is sufficient to ripen wheat properly. Not only so, but it 
was found that the wheat crop of 1864 ripened well with only the averafje 
temperature of 54"4°. In that year, however, the sunshine was much above 
the average, and the mean of the daily maximum temperature was high, being 
as high as in August 1861, when the mean temperature was 57'4°. 
It is probable that the longer time the sun is above the horizon in 
Scotland, as compared with Germany and France, renders the ripening of this 
cereal jjossible with a lower mean temperature, and when this is combined 
with a clear dry atmosphere, and consequently a blazing, scorching sunshine, 
grain of excellent quality is riisened, though the mean temperature rise no 
higher than 54'4°. From this it is clear that in regarding the influence of 
temperature on bringing plants to maturity, it is not mean temperature merely, 
but the way in which the vital element is distributed through the day and 
night, particularly at the critical periods of the plants' growth, which must be 
considered. A high mean temperature, with little variation, imjDlies a com- 
paratively low day temperature ; and, on the other hand, a moderately low 
mean temperature, with a large daily range, implies a high day temperature ; 
so that a climate with a comparatively low mean temperature may yet afford 
the warmth required in carrying on the higher functions of the plant which 
another climate of a higher mean temperature could not supply. 
Now, that which in the highest degree determines the mode in which 
temperature is partitioned throughout the twenty-four hours of the day is the 
amount of cloud and the degree of moisture in the atmosphere ; for a knowledge 
of which we must look to the rainfall through the months of the year as 
furnishing the best available key. 
The rainfall affects plants directly through the nourishment it conveys to 
them, and indirectly through the state of the sky which its amount or absence 
implies. Indeed, so great is the influence of rainfall on vegetation that we 
cannot be far wrong in regarding it as co-ordinate with tliat of temijerature. 
Whatever the law may be which expresses the atmospheric conditions that 
determine the limits of the growth of species, it must include in its functions 
both the heat and moisture of the air. 
Decandolle deduced the law for the distribution of species over a region 
whose climates are marked off from each other rather by variations of tempe- 
rature than of moisture. He then endeavoured to extend it so as to account 
for the distribution of the flora? of other regions, the climates of which may be 
characterised either as moist at all seasons or subject to marked variations of 
moisture at stated seasons. Perhaps not the least valuable of the results 
arrived at by him is the negative one stated in these words : — "On the borders 
of the Mediterranean Sea, the limits appeared so often determined by the 
humidity, or by causes still unknown, that the operations of temperature 
always et^caped my calculations." 
It may be predicted that when the limits of species have been drawn with 
some exactness for Central and Northern Europe, the regions from which 
Decandolle took his examples, they will be found to coincide with no mere 
temperature lines, however calculated and determined, inasmuch as there are 
much greater differences in the climates of this region than are generally sup- 
posed, as regards the rainfall, particularly in the manner of its distribution over 
the year.* 
The practical result of these views seems to be that the same 
summer temperature (I mean the average temperature of each 
twentj-four hours) may be arrived at in two ways — 
* ' Transactions and Proceedings of the Botanical Society of Edinburgh,* vol. 
vol. xi. Part II., 1873, pp. 262-264. 
