112 FOURTH REPORT 1834. 



from which each is formed, under the necessary external con- 

 ditions*. 



The recent experiments and observations of Ehrenberg have, 

 however, tended to increase our doubts concerning the validity 

 of these conclusions. He has not succeeded, as Spallanzani con- 

 ceived he did, in producing definite forms of animalcules from 

 definite infusions. On the contrary, he has found the forms to 

 vary under circumstances the most similar. He has explained, 

 however, how it is that Spallanzani might be mistaken in his 

 conclusion. The species pass through many gradations of form 

 in their progress to maturity, each of which forms may have 

 been readily mistaken for a distinct species. These are not so 

 very numerous ; but the rate at which the individuals are multi- 

 plied is altogether extraordinary. For instance, the Hydatina 

 *ew^a, which was observedfor eighteen days, is capable of a fourfold 

 increase in twenty-four hours, which may give more than a million 

 of individuals from a single ancestor in ten days, and on the most 

 moderate computation may give nearly seventeen millions in 

 twenty-four days. According to Ehrenberg, infusories exist in 

 all waters, (except rain and dew, in which he could not discover 

 them,) and in some parts of plants, though here, probably, only 

 in a diseased state of the plant. Further, he has succeeded in 

 detecting a complex organization in those animalcules lately 

 considei'ed of so simple a form. Even in the Monas, a creature 

 not more than the twenty-thousandth part of an inch in diame- 

 ter, the stomach is found to be of a compound structure, and 

 its motions are effected by cilia. In others he observed ova, 

 and propagation by means of them. If then, in the infusions 

 of Treviranus and Needham, no animalcules were produced 

 when the vessel was hermetically sealed, and the necessary 

 quantity of air exposed to a high heat — if they were produced 

 when fresh air was introduced after boiling — if the animalcules 

 have been shown to be capable of producing ova, which, indeed, 

 was jiever denied ; it seems more reasonable to suppose, that 

 those observers who did not succeed in discovering the complex 

 structure of the creature so extremely minute, might fail also 

 in discovering the first ova, though they really existed in the 

 infusion, than that the animal should arise spontaneously. 

 Ehrenberg has not succeeded in detecting these first ova. No 

 very violent improbability is included in the supposition that 

 bodies so infinitely small have been conveyed by the air, like the 



• Vide a full critique of the experiments of previous authors, and an account 

 of his own, in Treviranus, Biolor/ie, ii. 267, 403. 



