306 THE FLOATING-MATTER OF THE AIR. 



iDfusion — 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 labelling 

 them ' negative ' results. This, as shown by Dr. 

 William Eoberts, is an exact inversion of the true order 

 of the terms positive and negative.* 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 scru- 

 tiny, fully carried out, would infallibly lead him to the 

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

 in favour of spontaneous generation crumbles in the 

 grasp of the competent inquirer. 



The botanist knows that different seeds possess 

 different powers of resistance to heat.^ Some are 

 killed by a momentary exposure to the boiling tem- 

 perature, while others withstand it for several hours. 

 Most of our ordinary seeds are rapidly killed, while 

 Pouchet made known to the Paris Academy of Sciences 

 in 1866, that certain seeds, which had been transported 

 in fleeces of wool from Brazil, germinated after four 

 hours' boiling. The germs of the air vary as much 

 among themselves as the seeds of the botanist. In 

 some localities the diffused germs are so tender that 



' See his truly philosophical remarks on this head in the British 

 Medical Journal, 1876, p. 282. 



' I am indebted to Dr. Thiselton Dyer for various illustrations 

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

 high scientific importance should not have been more thoroughly 

 explored. Here the scoundrels who deal in killed seeds might be 

 able to add to our knowledge. 



