358 Developmental History of Infusorial, Animal Life. 



hundreds of observers have brought their contributions to the 

 general stock, and the modes of development of plants and 

 animals have been more and more clearly traced ; and each 

 extension of knowledge in this direction has had the effect of 

 narrowing the ground on which the spontaneous hypothesis 

 could possibly find a footing ; and the question now comes to 

 this : ' ' Is it more probable that a law of generation which is 

 well nigh universal in the organic world, should have an excep- 

 tion ; or that our researches have as yet been so faulty that 

 we have not as yet been able to bring this seeming exception 

 under the law '( One after another, cases which seemed excep- 

 tions have turned out to' be none at all. One after another, 

 the various obscurities have been cleared away, and it is there- 

 fore the dictate of philosophic caution which suggests that so 

 long as we remain in positive ignorance of the actual process, 

 we must assume that a general law prevails/''* 



Positive evidence would at once settle the dispute, and 

 I take this opportunity of directing ardent and aspiring 

 microscopists to the question, as one well worthy any amount 

 of time and trouble ; but I must tell them beforehand that 

 every one who has hitherto made any experiments, or atten- 

 tively followed those of others, has found it exceedingly diffi- 

 cult to devise any experiment which shall be conclusive. This 

 arises, first, because the facts elicited admit of very different 

 interpretations ; and secondly, from numerous sources of error. 

 " For it is quite true that there are organic beings of which 

 we can as yet only say that there is the strongest presumptive 

 evidence against their being exceptions to the otherwise uni- 

 versal law to which I have alluded. As an instance, we do 

 not know how the Amoeba arises, no one has even seen its 

 eggs, or ever been a witness of its mode of reproduction ; and 

 yet we find the Amoeba in almost every drop of rain-water, 

 and most vegetable infusions, so that we may be perfectly 

 certain that its ova are carried about by every breath of air." 

 Schultze, of Berlin, devised an experiment which might have 

 been thought to settle the question. This experiment proved 

 that an infusion of organic substance, supplied with air driven 

 through a strong acid, could be suffered to remain for three 

 months without either any animal or vegetable life becoming* 

 apparent. At the end of that time atmospheric air was allowed 

 to enter freely, and in three days the infusion was found to be 

 swarming with animalcules. 



It will be at once observed that the essential condition in 

 such an investigation is to be quite certain that no organic 

 germs are introduced into the liquid from without, and that 

 there should be secured a free supply of air, carrying no 



* Lewes. 



