ZOOLOGY. 393 



remain in positive ignorance of the actual process, we must assume that in 

 this case also the general law prevails. 



Positive evidence would of course settle the dispute; but every one who 

 has made any experiments, or has attentively followed the experiments of 

 others, will admit that it is excessively difficult to devise any experiment 

 which shall be conclusive, the facts elicited admit of such different inter- 

 pretations, the avenues by which error may enter are so numerous. I will 

 not narrate here the experiments of Fray, Gruithuisen, Burdach, Baer, and 

 others, since they cannot withstand serious discussion; nor will I adduce my 

 own, for the same reason. But those recently made by M. Pouchet have a 

 moi-e imposing character, and demand the strictest examination. 



The reader will observe that the cardinal point in the investigation is to 

 be certain that no organic germs could by any possibility be present in the 

 liquid which is to produce the animalcules. On the hypothesis that the ani- 

 malcules, like other animals and plants, are produced from germs or egizs, 

 these germs must be excessively minute, and easily overlooked. If they 

 exist, it is in the water and the air, awaiting the proper conditions for their 

 development. Supposing them to be floating about in the air, under the 

 form of dust-like particles, they would fall into, or enter, any vessel contain- 

 ing organic matter in a state of decomposition, and there develop; as the 

 deposited eggs of the insect developed in the decaying body of the dog. 

 Now, inasmuch as the presence of atmospheric air is one of the indispensa- 

 ble conditions of vitality, and without it the animalcules could not develop 

 and live, the initial difficulty is how to secure the presence of this air, and 

 yet be sure that the air itself does not bring with it the germs of the animal- 

 cules which we find in the liquid. Schultze of Berlin devised an experiment 

 which was thought to have finally settled this point, and to have refuted the 

 hypothesis of spontaneous generation. An account of this experiment, to 

 be found in the Edin. New Phil. Jour., for October 1837, shows that an infu- 

 sion of organic substances, supplied with atmospheric air, but not with an air 

 containing living germs, was suffered to remain thus from the end of May 

 till the beginning of August; but, during the whole of that time, no plant 

 or animal was developed in the infusion. The apparatus was now removed 

 from the flask, atmospheric air was allowed to enter freely without first 

 passing through acid or potassa solution and in three days the infusion 

 was swarming with animalcules. 



This really looked like a conclusive experiment. Xo sooner were measures 

 taken which would destroy the germs, supposed to be suspended in the at- 

 mosphere, than the infusion was kept free from animalcules ; no sooner was 

 the air allowed to. enter the flask in the ordinary manner, than animalcules 

 abounded. The proof did not, however, seem to me quite rigorous. It was 

 by no means clear that the air, in its passage through sulphuric acid, would 

 not suffer some alteration, perhaps electrical, affecting its vital properties; 

 and this doubt seemed confirmed by the experiments of M. Morrcn, com- 

 municated to the French Academy, May 22, 18->4; from which it appeared 

 that air, having passed though sulphuric acid, was incompetent to sustain 

 life, since the animalcules subject to it died in a few days. But M. Pouchet 

 announces experiments which, if correct, not only scatter this doubt, and 

 M. Morren's confirmation, but point-blank contradict the experiment of 

 Schultze. He declares that in following Solmitzc's experiment in every par- 

 ticulai", and also in repeating it with fresh precautions, he can constantly 

 exhibit animalcules and plants developed in an infusion in which every 



