cvi THE BEGINNINGS OF LIFE, 



impossible to say what amount of metamorphosis may take 

 place. Analogical evidence appears to be of no avail when 

 applied to such utterly variable creatures as these. We are 

 almost at the mercy of individual observers, and it seems 

 almost impossible for any of their statements to be checked 

 by saying that such or such a developmental cycle is an im- 

 possible one. From what we already know on the best 

 evidence, we obtain a basis of so shifting a nature, that he 

 would be a bold man who would dare to say, on a priori 

 grounds, that any particular, alleged, developmental meta- 

 morphosis could not take place. Almost everything must 

 turn upon our faith in the accuracy of the observer. 



Many other very startling modifications, to which I have 

 not alluded, have been described amongst the Infusoria, in 

 which, more especially after a process of encystment, one 

 form has given place to another, belonging even to what has 

 been reputed a totally different genus 1 . 



Such statements are, moreover, to a great extent, counte- 

 nanced by the fact that in organic solutions of a given kind 

 there is almost always a definite order of succession observ- 

 able amongst the Ciliated Infusoria which it contains. This 

 is a matter of notoriety, and most of the other explanations 

 which have been offered are certainly by no means con- 

 vincing. To give one example of such succession, we 

 may quote Dr. Arlidge's 2 account of Cohn's observations. 

 He says : ' In a vessel containing decomposing Spirogyra, 

 at first appeared countless specimens of Paramecium aurelia; 

 these were replaced by the Proteus of Baker, either the 

 Lachrymaria proteus or the Trachelocera olor (Ehr.) ; these 

 in their turn were followed by Chilodon cucullus, and after 



1 See the observations of Pineau in 'Ann. des Sc. Nat.,' 1848, 3 S6r. 

 p. 99 ; of J. Haime, in 3 Ser. vol. xix. ; and of Gros in 'Bullet, de la 

 Soc. de Natural, de Moscou.' 



2 Pritchard's ' Infusoria,' 4th ed. p. 372 ; or Cohn in Kolliker und 

 Siebold's ' Zeitschrift,' 1851, p. 258. 



