5 $4 VRA GMENTS F SCIENCE. 



edge, will he not substitute for the adventurous conclusion 

 that an organic infusion is barren at one place and sponta- 

 neously generative at another, the more rational and 

 obvious one that the atmosphere of the two localities which 

 have had access to the infusion are infective in different 

 degrees? 



As regards workmanship, moreover, he will not fail to 

 bear in mind, that fruitfulness may be due to errors of 

 manipulation, while barrenness involves the presumption 

 of correct experiment. It is only the careful worker that 

 can secure the latter, while it is open to every novice 

 to obtain the former. Barrenness is the result at which 

 the conscientious experimenter, whatever his theoretic 

 convictions may be, ought to aim, omitting no pains 

 to secure it, and resorting only when there is no escape 

 from it to the conclusion that the life observed comes 

 from no source which correct experiment could neutralize 

 or avoid. 



Let us again take a definite case. Supposing my 

 colleague to operate with the same apparent care on 100 

 infusions or rather on 100 samples of the same infusion 

 and that 50 of them prove fruitful and 50 barren. 

 Are we to say that the evidence for and against heterogeny 

 is equally balanced? There are some who would not only 

 say this, but who would treasure up the 50 fruitful flasks 

 as "positive" results, and lower the evidential value of 

 the 50 barren flasks by labeling them " negative " results. 

 This, as shown by Dr. William Roberts, is an exact in- 

 version of the true order of the terms positive and nega- 

 tive.* Not such, I trust, would be the course pursued by 

 my friend. As regards the 50 fruitful flasks he would, I 

 doubt not, repeat the experiment with redoubled care and 

 scrutiny, and not by one repetition only, but by many, 

 assure himself that he had not fallen into error. Such 

 faithful scrutiny fully carried out would infallibly lead him 

 to the conclusion that here, as in all other cases, the 

 evidence in favor of spontaneous generation crumbles in 

 the grasp of the competent inquirer. 



The botanist knows that different seeds possess different 

 powers of resistance to heat.f Some are killed by a 



* See bis truly philosophical remarks on this head in the " British 

 Medical Journal," 1876, p. 282. 



f I am indebted to Dr. Thiselton Dyer for various illustrations of 

 such differences. It is, however, surprising that a subject of such 



