who have entered upon it from Needham's time to the present day ought to have made use of it.
The serious and grave difficulty, on which, during this period, all discussions raised between heterogenists and panspermists have turned, is so to arrange the experiments as to remove every suspicion of the intervention of germs brought from without, or preexisting in the liquid.
If the result is negative, if when all precautions that seem to be necessary have been taken, and all causes of error have been removed, there is no formation of infusoria, it will be difficult to raise any serious objection to the inevitable conclusion, provided that the methods employed for the purpose of eliminating the preëxisting germs are not of such a nature as to modify the medium, and to render it unfit for the development and the nutrition of living organisms. If, on the contrary, we still meet with the birth of living beings, the suspicion will always revive that the experiment has been badly performed, and that a contrary result would have been obtained by conducting it more carefully. The heterogenists, therefore, find themselves in a more disadvantageous situation than their opponents, and, notwithstanding the success which they may obtain, they will never convince them.
We think, therefore, that it is useless to give here a detailed account of their minute researches; they must be consulted in the original memoirs. A single experiment which proves, by a negative result, that organic infusions, protected from germs from without, do not give birth to infusoria, is worth more, scientifically speaking, than ten experiments tending to establish the contrary opinion.
If, therefore, we pass over the details of the fundamental experiments of the heterogenists, and speak of those the results of which are conformable to the ideas of the panspermists, it will not be in a spirit of partiality. We are convinced that the latter are the only ones free from all objections, the relative skill of the operators being disregarded, and considered as nothing in the estimate formed. We may, however, say that M. Pasteur's researches may serve as a model for all those who may wish to conduct investigations of this kind, whatever may be the preconceived opinion by which they are guided. By their precision, and the care taken to remove every source of error, they leave nothing to be desired.
As the results obtained by M. Pasteur lead him to deny spontaneous generation, his opponents must above all prove that he is mistaken, by adopting the same rigorous experimental conditions. Needham's experiments, which led him to admit and sustain the doctrine of spontaneous generation, consisted essentially in placing organic substances which were capable of decomposition, in vessels hermetically sealed, which were subsequently submitted to a high temperature, in order to destroy the preexisting germs. The work of the English writer attracted great notice on account of the support of Buffon,
