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, a 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 within Physiology}. The formation of Infusories is 
no primary production of organic matter. Their immediate origm 
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*® the same obscurity recurs, and the difficulty of 
applying the proposition that all living creatures come from eggs 1s 
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, im 
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 HHRENBERG. His 
travels in Asta 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. 
2 « Bs giebt keine Erfahrung, die fiir eine Entstehung lebender Korper aus Stoffen der 
leblosen Natur spriche.” G. R. TREvinANUS, Biologie, 11. 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. 264—353. 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 
Infusories, Chlamidomas pulvisculus (HHRENB. L. I. p. 64), Huglena viridis (HHRENB. 
p- 110), &e. 
3 [The presence of Hntozoa in situations where it was thought impossible they could 
be introduced from without is now explained: vd. notes on that class. | 
— —_—-~—~— ss 
