584 F&AGMKNTS OP SCIKNC& 
edge, will he not substitute for the adventurous conclusion 
thut 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 mindj that fruitful-ness 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 lieterogeny 
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 Eoberts, 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 his 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. 
