834 
THE PRINCIPLES OF BIOLOGY. 
(D.)—Differences of climate— 
Latitude. 
Extreme temperatures in summer and winter. 
Altitude. 
Humidity. 
(E.)—Differences arising from the presence or absence of 
other plants. 
(F.)—Differences arising from the presence or absence of 
certain animals and especially of insects. 
Influences of the above nature are called by Herbert 
Spencer “ Negative.” What is meant is that the absence 
of one or more of the above conditions will determine the 
absence of any given plant, but the presence of those condi¬ 
tions will not necessarily determine the presence of such 
plant. We are thus led to the consideration of the second 
of the main facts of distribution, viz., that similarity of 
conditions is frequently accompanied by great dissimilarity 
of Fauna and Flora. 
The explanation is that the Fauna and Flora of any area 
on the earth’s surface is derived from that which immediately 
preceded it in time, and that similar variations of condition 
acting on dissimilar organic materials have produced dissimilar 
results. The earth’s surface has been in a state of continuous 
modification throughout past ages, and corresponding modi¬ 
fications have occurred in its organic inhabitants. The 
present phenomena of distribution are the result, and their 
study enables us to read more readily the past history of the 
earth. 
Before the distribution of plants can be used as a key to 
that history, we must understand the various means by which 
distribution is effected or we may fall into serious error. 
1st.—A number of plants are distributed all over the 
world by the agency of man. They are generally of intense 
vitality, of very unprepossessing appearance, and not only of 
no economic value but great hindrances to cultivation. Such 
are the thistle, dock, and nettle, and other noxious weeds. 
The way in which these plants colonise a foreign country may 
be seen by a glance at the Flora of New Zealand. The 
Canterbury Province in the Southern Island contains 750 
native phanerogamous plants. Within the last twenty years 
250 European species (one third in number of the indigenous 
Flora) have been introduced. They are, many of them, 
English weeds, and are increasing witli such rapidity as to 
threaten the native plants with extirpation. 
