488 The Controversy on Spontaneous Generation : [Oct., 



stances, become mouldy, it will not undergo that change if pre- 

 viously boiled and filtered — but it must in fairness be added that 

 he says, even if exposed to the air. Whether he means constantly 

 exposed to the air, or only occasionally, I am unable to say. How 

 such substances become mouldy will be seen presently, and it will 

 be found to have a direct bearing on the argument. 



Before proceeding to describe my own recent investigations, 

 however, I desire to make one more reference to the published 

 opinions of Dr. Bastian, to show how necessary it is to be cautious 

 before we construe the microscopical appearances connected with this 

 inquiry. 



In speaking of the pellicle which appears on the surface of 

 infusions, Dr. Bastian says,* "What Burdach named the pro- 

 ligerous pellicle of organic solutions, is made up of an aggregation 

 of monads and bacteria in a transparent jelly-like stratum on the 

 surface of the fluid. It constitutes at first a thin scum-like layer, 

 and although the monads and bacteria entering into its composition 

 are motionless, M. Pouchet and others were not warranted in 

 assuming from this fact alone that they were dead. There is 

 indeed good reason for believing to the contrary, since, as pointed 

 out by Cohn, when any of these particles are set free from the 

 broken edge of a pellicle, they always resume their movements. 

 Motion, therefore, may simply be prevented by the presence of the 

 transparent jelly-like material in which they are imbedded, although 

 the particles may be undoubtedly living." 



Under what circumstances the observers examined this so-called 

 "proligerous pellicle," I am unable to say, and Dr. Bastian him- 

 self says, that owing to his observations being carried on in winter, 

 he was not able to witness those changes observed by Pouchet ; but 

 he describes certain other changes in this pellicle on infusions 

 which, according to his account, resulted in the development of 

 unicellular organisms. 



Now, with all deference to the eminent observers quoted by 

 Dr. Bastian, I venture to say that the appearances referred to have 

 no bearing whatever upon the controversy, inasmuch as they are 

 by no means confined to infusions. 



Long before I had heard the expression " proligerous pellicle," 

 or was aware that this phenomenon was supposed by the advocates 

 of heterogenesis to precede the creation, de novo, of living forms, I 

 had myself observed a precisely similar appearance in pure distilled 

 water exposed to the atmosphere. This was recorded at the time, as 

 follows, in a paper read before Section D of the British Association 

 in 1863 :— 



" Let me, however, briefly refer to the results of the exposure of 

 distilled water only, in July, for that experiment has not been re- 



* ' Nature,' June 30, p. 172. 



