THE EVOLUTION OF PLANTS. 
19 
partially succeeded with lignified tissue (but the solution of this difficulty 
is only a question of time), we are in a position to say that we regard the 
occurrence of ' connecting threads ' as universal ; and, with the excep- 
tion of the lignified elements above referred to, we are able to demonstrate 
the threads in any part of the plant, including the tissues of embryos, 
and stem and root-growing points. 
The conduction of stimuli and the quick passage of watery solutions, 
food, and the like appears to take place via the threads and across the 
pit-closing membranes." 
With these remarks I leave heredity and return to my principal 
factors of evolution. The first and the most fundamental was 
Vaeiation. 
Variation is quite a familiar thing to every student of nature, and I 
rather think that its present activity provides the systematic botanist 
with by far the larger part of his work. You may remember the aphorism 
that no two blades of Grass are alike. Variation is important, because 
it is the beginning of all evolution possibilities, whether in nature or in 
the garden. It provides better or worse forms for competition in the 
struggle for existence. Variation begets variation. All variation of 
environment, especially in the matter of food supply, tends to produce 
variation in the organism. Consider, for instance, the Groundsels and 
Spurges of South Africa ; how they differ from the Groundsels and 
Spurges of Europe. By crossing and hybridising variation must arise, 
as indeed we know. All this is fact, but theorising does no harm. If 
we go to the very beginning of things we have perhaps no right to 
assume that life plasma must have had originally the power of exact 
reproduction. The power of approximation to the parent may indeed 
have been an acquired character. Sex itself may have evolved, some 
think, in great measure, for the express purpose of producing variation. 
Weismann refers all variation to sex. Certainly without sex variation 
could not have been what it is. It is very likely that variation is pro- 
duced by unknown causes. It appears to be a property of the organism 
for when the directive influence which keeps up a certain amount of 
stability is removed, variation at once appears. It is said that in this 
way we obtain variation in our domestic animals — pigeons, for instance — 
and the principle is called by Weismann " panmixia." The stability of 
which I speak may be noteworthy in the case of Egypt, where for long 
periods, it is said, vegetation has remained unaltered. At any rate such 
a condition is easily explained. The directive influence of climate has 
remained the same, and nothing has disturbed the balance. A new 
influence, a new mould provided, and the vegetation of Egypt would no 
doubt be found to vary and to flow into that mould. 
Secondly, we have the 
Struggle for Existence. 
There is perhaps no phenomenon of nature that is at once so important 
and so universal as the struggle for existence. Many more plants are 
born than can by any chance survive. If a tree that lives a thousand 
years were to produce an offspring that grew to maturity once in a 
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