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 



