18 THE STIDY liF I'l.AXTS IN ANCIENT AND MODERN TIMES. 



the moleculiir structure of tlie cell-ineiubrane by arguing from the phenoiueiia 

 inanifested by that membi'aoe; when we investigate the meaning of the pecuHar 

 thickenings and sculpturings of the walls of cells, or when we discover the sti'ange 

 forms of flowers and fruits to be mechanical contrivances adapted to the fomis 

 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 uin-e- 

 strained, or without the steadying ballast of actual observations, it has frequently 

 led its followers into eri'oi-, does not detract at all from its extreme value as an 

 aid to research, notwithstanding the fact that it is responsible for tlie wonderful 

 fantasies of nature-philosophy of which a few specimens have been gix-en. 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 regai-d with contempt the ideas of plant- 

 life formed by our predecessoi's. It should never be forgotten how much smaller 

 was the number of observations upon which botanists had to rely in former times, 

 and liow much less perfect wei-e 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 underljäng 

 them; and attempts are made to grasp the nature of such uniformitj' 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 appeal's. 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 multiplj-; 

 .scientific instruments become more delicate, and some of the newly-observed facts 

 will not adapt themselves to the scheme of the earlier generalization. At fii-st 

 they are held to be exceptions to the rule. By degrees, however, these exceptions 

 accumulate; the law has lost its univei-sality and must undergo expansion, or else 

 it has become quite obsolete and must be replaced by another. So it has been 

 in all ])ast 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. 



The remarks on the limitations of our knowledge of nature, the importance 

 of imagination as an aid in research, and the variability of our theories were made 

 v.ith a view to moderate, on the one hand, the exuberant hopes raised by the belief 



