REPORT OX ANIMAL PHySIOI.OGY. Ill 



animal and vegetable form. If the facts were really thus, then 

 might the objection to the hypothesis of metamorphosis founded 

 on the permanence of existing forms be encountered. It might 

 be averred that, notwithstanding our ignorance of the means, 

 the necessary conditions for such successive changes may have 

 been supplied in the earlier periods of the world, at epochs so 

 far removed, that the few thousands of years which have passed 

 away since the appearance of man upon the globe bear no pro- 

 portion to their immense distance, and only show that the rate 

 according to which the conditions of change are produced is a 

 very slow one. 



Let us see to what conclusion the latest obsei'vations on In- 

 fusoria are tending. 



It is well known that the experiments of Redi and of Vallis- 

 nieri were considered to have refuted the notions of the ancients 

 concerning spontaneous generation, until those of Tuberville 

 Needham, of O. F. Miiller and of Wrisberg, performed with the 

 most considerate exclusion (if that be possible) of circumstances 

 likely to throw a doubt upon the result, revived them. Miiller, 

 repeating the experiments of Needham, concludes, that animal 

 and vegetable matter, by solution in water, is reduced to minute 

 membranous shreds, upon which, in a short time, are seen micro- 

 scopic globular points. These enter into a tremulous motion, 

 which gradually becomes more apparent ; the globules are de- 

 tached, and Infusoria are produced from them. These first Infu- 

 soria, he says, abound in all fluids, and are not to be confounded, 

 as is usuallj^ done, with other Infusoria, being, on the contrary, 

 elements which are the component molecules of all animals and 

 plants. 



The conclusions Spallanzani drew from his experiments were 

 opposed to those of the above-named naturalists. He found the 

 structure of the infusory animals to vary with the nature of the 

 infusion, and explained their appearance upon the supposition 

 that ova had been introduced with the animal matter, or had 

 been suspended in the air, whose admission, at least in some 

 degree, is necessary for Ihe success of the experiment. 



The experiments of Priestley, of Ingenhouz, of Treviranus, 

 appeared to prove that the green matter of Priestley, produced 

 in organic infusions on exposure to light, is first a mass of ani- 

 malcules ; then is resolved into green globules, which concrete 

 into confervse ; then, after the solution of these, again becomes 

 infusory animals and vegetables of a larger form. The organic 

 particles appeared indestructible, and conmion to each form of 

 life, passing from one to the other, and supplying the substance 



