123 MONTAGNE ON 



Oidium to advance au opinion that this mould is capable of being 

 transformed into four other fungals belonging to different tribes ; 

 viz., Penicillium candidura, Trichothecium roseura, Alternaria 

 tenuis, and Cladosporium Fumago. 



Without suggesting doubts as to the exactness of the facts 

 observed, we may contest their interpretation. On points of such 

 delicacy and difficulty, the best observers are subject to deception. 

 It is not long since another distinguished observer, but not a 

 mycologist, and mycology is a more difficult science than is 

 frequently imagined, attempted experiments on an analogous 

 subject, taking, as his point of departure, the curious observation 

 of a mycelium found by M. Rayer in a hen's egg, the evolution of 

 which gave rise to a new Dactylium ; * experiments which led to 

 consequences as singular, and to which it may be permitted to 

 oppose, if not an absolute incredulity, at least a prudent degree of 

 doubt. 



In experiments of this kind, it is well to be assured that 

 nothing is more difficult, or even, I might say, impossible, than 

 to guard against every source of error. It is not as in experi- 

 ments with higher plants, where one can see what one is about. 

 We believe that we have sown a spore, where nature, without our 

 knowledge or without its being possible to guard against it, has 

 substituted one totally different.f In other cases the error arises 

 from the fact that the spores of the different species which we 

 imagine to px'oceed from one another, are propagated at the same 

 time because they inhabit at the same time the same matrix, I 

 cannot indeed indicate all the sources of error. It is not suffi- 

 ciently remembered that the atmosphere is a vast receptacle 

 where innumerable spores of every kind float together, mixed and 

 invisible, and which wait only the peculiar circumstance necessary 

 for their individual development. 



Whatever one may do, or whatever precautions may be taken, 

 we cannot avoid this substitution of one spore for another on 

 which we wished to make experiment. If, therefore, with my 

 own eyes I were witness to the extraordinary metamorphoses in 

 question, far from giving them credit I should not even try to 

 give an explanation. We have in fact aiTived at a period when 

 people wish at all risks to speak of themselves, and when simple 

 observations do not satisfy the mind unless they are foi'ced beyond 



* Rayer, Arch, de Med. Comp., 1843, p. 175 ; and C. Robin, 1. c. p. 543. 

 + See Coinptcs Rendus, Aug. 18, 1836. Annates de la Soc. sericicole, 

 1847. Robiu, 1. c. p. SCO, 603. 



