42 CLASS I. 



means necessary to connect such a conception as this with the term 

 equivocal generation. As long as it is not pretended by this term 

 to afford an explanation, but only to indicate that there are some 

 animal and vegetable species that arise not from eggs, but, in a 

 way that we are unable to explain, from the decomposition of organic 

 matter, so long do we believe that the expression cannot at present 

 be dispensed with in Physiology *. The formation of Infusories is 

 no primary production of organic matter 2 . Their immediate origin 

 from the organic matter of Infusions has never, as we believe, been 

 observed at the very instant of its occurrence, and probably never 

 will be. Even in the development from the egg we never see the 

 forming, but only the thing already formed. In the case of the 

 intestinal worms 3 the same obscurity recurs, and the difficulty of 

 applying the proposition that all living creatures come from eggs is 

 but too obvious from the very constrained and improbable explana- 

 tions which have been resorted to. The reason why organisable 

 matter assumes those determinate forms that are distinguished as 

 genera and species, is altogether unknown : and Physiology is, in 

 the same degree, unable to explain how it is that in a complexly 

 organised creature developed from cells, in one part muscular fibre 

 should arise, in another nerves, and cartilage in another. 



The knowledge which we possess of the geographical distribu- 

 tion of Infusories is due to the investigations of EHRENBERG. His 

 travels in Asia and in Africa have taught us that in different coun- 

 tries different species, nay different genera of these animals are 

 found. The species which have the widest geographical distribu- 

 tion in the northern hemisphere are Monas termo, Uvella glaucoma, 



1 Vide note 2, page 40. 



* " Es giebt Tceine Erfahrung, die fur eine Entstehung lebender Korper aus Stoffen der 

 leblosen Natur sprache." GL K. TREVIRANUS, Biologie, n. s. -266. In this work may be 

 found a full account of the earlier observations on this subject, to which the author has 

 added many investigations of his own, s. 264353. Although more than forty years 

 have since elapsed, the labours of TREVIRANUS on this point still retain a great value. 

 As to the green matter of PRIESTLEY, in which transformations of infusories are sup- 

 posed to occur, this is not exclusively of a vegetable nature, but consists, according to 

 the exact investigations of later enquirers, of a collection of dead, and in part still living 

 Infitsories, Chlamidomas pulvisculus (EHRENB. L. i. p. 64), Euylena viridis (EHRENB. 

 p. no), &c. 



3 [The presence of Entozoa in situations where it was thought impossible they could 

 be introduced from without is now explained : vid. notes on that class.] 



