April 12, 1883] 
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mastered, to show what general conclusions may be 
drawn from them. The student will thus not merely ap- 
preciate the mastery which a comprehensive point of 
view gives of the subordinate facts, but he will get some 
insight into the value of the evidence upon which the in- 
duction rests, and be quite prepared to understand that in 
the face of a wider survey of observations it may have to 
be materially modified. This method of procedure seems 
to me to be not only the scientifically sound one, but to 
have an educational value of a very high order. 
The opposite method is to start with the general 
principles and derive the explanations from them. This 
no doubt affords play for ingenuity. But the intellectual 
discipline is immensely inferior. And when the elaborate 
structure is built up, it is impossible not to begin insensibly 
to resent with jealousy any criticism of its foundations, 
even when it has become difficult to resist the suspicion 
that they are decrepit. This state of things might be 
illustrated from the history of the biological sciences 
again and again. Generalisations which at first were justly 
hailed with enthusiasm have finally hecome mischievous 
obstructions in the way of their adherents arriving at a 
better knowledge. 
I do not mean to say that I prophesy this fate for the 
evolution theory. But I confess I look with great dislike 
on the growing tendency, especially in writings intended 
for popular consumption, to explain everything by it 
deductively. We may think the probability of organic 
forms having been evolved is very great. But the how 
of the process is what in every case we have to prove. 
In this way the induction on which the theory of evolu- 
tion rests perpetually widens its base, while at the same 
time our detailed knowledge of the subordinate laws 
through which it acts continually accumulates. But if, 
assuming the truth of the evolution theory, we proceed to 
spin out of our heads an explanation of how any par- 
ticular phenomenon came about, I fail to see in what way 
we are the wiser. The theory of evolution runs a very 
good chance of being burlesqued; and at the best we 
find ourselves in possession not of a new knowledge, but 
merely of an ingenious literary exercise. 
In several successive articles, a very able writer, Mr. 
Grant Allen, has discussed and given a deductive explana- 
tion of the shape of leaves. Now this isa matter on which 
a good many botanists have probably bestewed much 
thought, and it is well known to be beset with immense 
difficulties. I believe I am justified in saying that for the 
last ten years of his life it constantly engaged the atten- 
tion of Mr. Darwin, and it cannot be doubted that if the 
problem had at all readily admitted of solution he would 
have at any rate made some attempt to clo for leaves what 
he did for flowers. In work of this kind Mr. Darwin 
assumed nothing. His method was purely inductive. He 
made an immense number of observations drawn from the 
most widely severed types existing under the most varied 
conditions, and he gradually felt his way towards some 
general conclusions. But the fact is that the form of 
leaves, in common with a great deal of external morpho- 
logy, is a product of a complex of conditions. Whatever 
general principles control it, we may be pretty sure that 
they do not lie on the surface. It is sufficient to mention 
a few of the obvious factors that must enter into the 
solution to see that this must be true. In the first place 
we have the conditions of development; a leaf which, 
like that of the wild hyacinth, has to be pushed up 
through compressed soil, must be shaped accordingly, and 
differently from one, such as that of a horse-chestnut, 
which languidly expands, like the wings of a butterfly 
newly escaped from its chrysalis, into the unresisting air. 
Then we have mechanical conditions; a leaf is a much 
greater feat of natural engineering than a stem; a 
fragile expanded structure has to be carried on a single 
support and supplied with a framework which must have 
the necessary rigidity not to collapse, and at the same time 
be carefully adjusted to withstand wind-strains. Then it 
must be adapted to meteoric conditions; it must be 
capable of withstanding solar radiation without being 
scorched, and its own reduction of temperature at night 
without being irremediably frozen. With this last circum- 
stance is probably correlated the great variety of nycti- 
tropic movements which leaves execute, and these again 
react on their form and construction. The enumeration 
might be very much prolonged; this is only a sample. 
But it will suggest to most people, as I imagine it did to 
Mr. Darwin, that, before asserting anything definite about 
the laws that govern the form of leaves in general, there 
is an enormous amount to be made out about their relation 
to particular circumstances of the environment. 
But, as far as I can make out, all these considerations 
count as nothing with Mr. Grant Allen. ‘“ Two points,” 
he says, “between them mainly govern the shapes of 
leaves.” One of these is the relation of the leaf to sun- 
light: and the importance of this no one doubts. The 
other is the tendency of the plant ‘‘to have its whole 
absorbent surface disposed in the most advantageous 
position for drinking in such particles of carbonic acid as 
may pass its way.” The importance of this, Mr. Grant 
Allen adds, ‘‘ appears hitherto to have been too frequently 
overlooked.” 
Now, as I have said, I think the deductive method is 
a bad way of solving morphological problems. It is still 
worse when the principle started from is more than doubt- 
ful. Mr. Grant Allen speaks of the competition of plants 
for carbonic acid as of the same kind as that of carnivo- 
rous and herbivorous animals for their respective foods. 
But it is surely nothing of the sort. Carbonic acid is an 
ingredient of the atmosphere to the extent of 1-2500th of 
its bulk. But only about one-quarter of the earth’s surface 
is occupied by land, and from this a large deduction may 
be made on account of areas incapable of sustaining 
vegetation. There is therefore an enormous reserve of 
atmospheric carbonic acid which, as the atmosphere is 
rarely at rest, is constantly brought within the range of 
vegetation. Moreover, the carbon which plays its part in 
vegetation is continually being released from its organic 
trammels and the secular accumulation of carbon in the 
soil, though the work of vegetation is at most extremely 
slow. On what possible grounds then can Mr. Grant 
Allen talk of a competition for carbonic acid, which the 
wind that “ bloweth where it listeth” perpetually and z7z- 
partially supplies to the tissues capable of absorbing it? 
It cannot be doubted that, fer uzt of absorbent surface, 
one plant in a locality will get as much carbonic acid as 
another, no more and no less. And when I say per unit 
of absorbent surface, I do not mean external surface, 
which, as wellas the shape, I apprehend has nothing to do 
with the matter. It is of no consequence how the chloro- 
phyll-containing cells which bound the air-passages are 
massed into a leaf, provided that there is enough of them 
to do the carbor-fixing work of the p!ant. When, there- 
fore, Mr. Grant Allen arrives at the conclusion that ‘‘the 
extent to which leaflets are subdivided depends upon the 
relative paucity of carbon in their environment,’’ I confess 
that I should much like to see the experimental data, if 
any, on which this statement rests. As there are plants 
which at different periods of their lives produce much 
and little divided leaves, the point would possibly admit 
of being actually tested. 
Now with regard to the submerged foliage of water- 
plants, I am free 1o admit that I think Mr. Grant Allen 
has made a point. These must absorb their carbonic acid 
superficially, being destitute of stomata and intercellular 
passages. But I do not see why he should say that the 
proportion of carbonic acid held in solution by water is 
very small. It is, I believe, never less than the proportion 
that occurs in the atmosphere, and may rise to nearly 
one per cent. 
W. T. THISELTON DYER 
