18 THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES. 
the molecular structure of the cell-membrane by arguing from the phenomena 
manifested by that membrane; when we investigate the meaning of the peculiar 
thickenings and sculpturings of the walls of cells, or when we discover the strange 
forms of flowers and fruits to be mechanical contrivances adapted to the forms 
of certain animals, and judge the extent to which these contrivances are advan- 
tageous, or the reverse, to the plants—in all these and similar investigations 
imagination plays a predominant part. Experiment itself is really a result of 
the exercise of that faculty. Every experiment is a question addressed to nature. 
But each interrogation must be preceded by a conjecture as to the probable state 
of the case; and the object of the experiment is to decide which of the preliminary 
hypotheses is the right one, or at least which of them approaches nearest to the 
true solution. The fact that when the imagination has been allowed to soar unre- 
strained, or without the steadying ballast of actual observations, it has frequently 
led its followers into error, does not detract at all from its extreme value as an 
aid to research, notwithstanding the fact that it is responsible for the wonderful 
fantasies of nature-philosophy of which a few specimens have been given. Nor 
should we esteem it the less because enlargements of the field of observation and 
improvements in the instruments employed have again and again led to the sub- 
stitution of new ideas for those which careful observers and experimentalists had 
arrived at by collating the facts ascertained through their labours. 
For the same reasons it is unfair to regard with contempt the ideas of plant- 
life formed by our predecessors. It should never be forgotten how much smaller 
was the number of observations upon which botanists had to rely in former times, 
and how much less perfect were their instruments of research. Every one of 
our theories has its history. In the first place a few puzzling facts are observed, 
and gradually others come to be associated with them. A general survey of the 
phenomena in question suggests the existence of a definite uniformity underlying 
them; and attempts are made to grasp the nature of such uniformity and to define 
it in words. Whilst the question thus raised is in suspense, botanists strive with 
more or less success to answer it, until a master mind appears. He collates the 
observed facts, gathers from them the law of their harmony, generalizes it, and 
announces the solution of the enigma. But observations continue to multiply; 
scientific instruments become more delicate, and some of the newly-observed facts 
will not adapt themselves to the scheme of the earlier generalization. At first 
they are held to be exceptions to the rule. By degrees, however, these exceptions 
accumulate; the law has lost its universality and must undergo expansion, or else 
it has become quite obsolete and must be replaced by another. So it has been 
in all past times, and so will it be in the future. Only a narrow mind is capable 
of claiming infallibility and permanence for the ideas which the present age lays 
down as laws of nature. 
These remarks on the limitations of our knowledge of nature, the importance 
of imagination as an aid in research, and the variability of our theories are made 
with a view to moderate, on the one hand, the exuberant hopes raised by the belief 
