INTERPRETATION OF NATURE. 39 
The confusion of theory and fact is a fault too commonly 
seen, especially, one regrets to say, among those whose 
business it is to teach the principles of science. 
For example. Everybody knows that many plants, when 
kept from the influence of light, turn pale, and by lengthening 
their internodes grow long. It is commonly taught that this 
is the plant’s adaptation for finding light. That the plant is 
devoting its energies to the production of extra long shoots, 
because by doing so it stands a better chance of thrusting 
some part of its person out of the darkness than if it grew 
normally. More than once I have heard it said, by people 
who ought to know better, that this groping for daylight is a 
fact. But is it? Of course not. 
Many plants become etiolated (lose their green colour) and 
lengthen out their members when the stimulus of light is 
withheld. This is the fact. It is obvious that by this abnor- 
‘mal growth their chances of reaching daylight are enhanced. 
This is also a fact. But the statement that they enhance their 
chances or any parallel statement is not a fact, and is probably 
(but not necessarily) untrue. 
The point is that because the chances of good fortune are 
increased by the plant’s behaviour, it has been inferred that the 
behaviour is a direct response to the existence of those chances, 
which inference is a theory accounting for the plant’s behaviour, 
and must not be confured with the fact of behaviour itself. 
Again, the study of cytological phenomena led to the 
prediction by Rabl of a remarkable form of conjugation of 
centrosomes. Fol, in 1891, a year after Rabl’s prediction, 
actually described such a conjugation in the growing cells of 
sea urchins under the picturesque name of ‘“ The quadrille 
of centres.” Later in the same year Guiguard discovered 
the same phenomenon in a member of the plant world (in a 
lily in fact). Between 1891 and 1895 various workers adduced 
additional examples from a snail, a trout, anda lancelet. It is 
now known that no such phenomenon exists. 
Many examples like the foregoing will occur to you, in which 
one clearly sees the influence of the preconceived idea. 4 
The tendency is to observe what theory predicts. Most of 
us have a theoretical axe or two to grind, and the difficulty 
