PREFACE. 5 



tions." And so it actually happened, for Maxwell 

 found by means of them "the very equations, whose 

 singular and almost incomprehensible power Hertz 

 has so beautifully portrayed in his lecture on the rela- 

 tions between light and electricity." "Maxwell's for- 

 mulae were the direct outcome of his mechanical 

 models." "These ideal mechanisms" — so relates Boltz^ 

 mann in the same interesting essay — "were at first 

 widely ridiculed, but gradually the new ideas worked 

 their way into all fields. They were themselves more 

 convenient than the old hypotheses. For the latter 

 could be maintained only in the event of everything's 

 proceeding smoothly; whereas now little inconsist- 

 encies were fraught with no peril, for no one can take 

 amiss a slight hitch in a mere analogy. — Ultimately 

 Maxwell's ideas were philosophically generalised as 

 the theory that all knowledge consists in the dis- 

 closure of analogies." 



But not only does it seem that there is little appre- 

 ciation among biologists for the scientific import of 

 imagination, they also appear to have little sense for 

 the significance of theor y. It is a favorite attitude 

 nowadays to look upon theory as a sort of superfluous 

 ballast, as a worthless survival from the epoch of 

 decrepit "nature-philosophies." People pronounce 

 with pride the miscomprehended utterance of New- 

 ton, Hypotheses non Ungo, and place the value of the 

 slightest new fact infinitely higher than that of "the 

 most beautiful theory."^ And yet theory originally 



lOf late this saying of Newton's is frequently quoted as 

 if Newton were a downright contemner of scientific hypothe- 

 ses. But if we read the passage in question in its original 

 context, we shall discover that his renunciation of hypotheses 

 referred solely to a definite case, viz., to that of universal 



