430 



GENERATION. 



some fishes and reptiles ; but the increased 

 knowledge of the structure and habits of these 

 animals, and in particular the observations of 

 Redi* and others, demonstrated the error of 

 this opinion, and shewed it to have arisen 

 merely from the circumstance of their real 

 mode of generation not having been observed. 

 After this many felt inclined to reject entirely 

 the occurrence of spontaneous generation in 

 any class of organized beings, and at the present 

 day the question cannot be regarded as by any 

 means entirely set at rest. From the nature 

 of the observations and experiments required in 

 an investigation of this nature, there is almost 

 an impossibility of arriving at a perfectly satis- 

 factory conclusion ; but so far as the facts at 

 present entitle us to form an opinion, it may be 

 stated that spontaneous generation, if it occurs, 

 takes place in the simplest kinds of organized 

 beings only ; that in most of them it is only 

 occasional ; and that therefore this form of ge- 

 neration is to be looked upon as a rare excep- 

 tion to the usual and almost universal mode of 

 reproduction by the separation of a living por- 

 tion from a parent body. 



Minute animalcules, the greater number of 

 which are so small as to be visible only with 

 the microscope, are formed in the infusions of 

 almost all kinds of organic matter, such as 

 starch, sugar, gum, seeds, and different animal 

 substances, when these infusions enter into pu- 

 trefaction. The kinds of these animalcules are 

 very numerous, and the circumstances which 

 seem to determine the formation of one or other 

 sort are infinitely varied. Thus the nature of 

 the substance suspended in the infusion ; and, 

 in the same infusions, the degree of heat, the 

 extent of the decomposition, the quantity and 

 nature of the air admitted, the rapidity with 

 which it is renewed, and the strength of the in- 

 fusion or the relative proportion of water and 

 organic matter in it, all appear to exert a certain 

 influence in determining the formation of one 

 or other of the kinds of animalcule. 



Two suppositions may be entertained regard- 

 ing the first origin of Infusoria ; the one, that 

 of their spontaneous generation ; the other, that 

 of their developement or evolution from some 

 pre-existing egg or germ. Those who disbe- 

 lieve in the first and adopt the second hypo- 

 thesis hold that the ova of the animalcules exist 

 in the substances of the infusions, or are float- 

 ing every where in the atmospheric air ; that 

 these ova become developed in that species of 

 infusion only which is suited to serve as 

 their proper nidus or matrix ; and that all the 

 varieties of animalculse in different infusions 

 depend upon the infusions being suited, from 

 their composition or the external agencies to 

 which they are subjected, to cause the develop- 

 ment of different sorts of ova. The supporters 

 of the hypothesis of Spontaneous Generation 

 hold, on the other hand, that certain changes 

 of composition of the organic molecules in the 

 infusions, in whatever way induced, are the sole 

 cause of the formation of one or other kind of 

 animalcule. 



* De Generatione Fnsectorum. Amst. 1686. 



Spallanzani,* one of the most strenuous op- 

 ponents of the hypothesis of spontaneous gene- 

 ration, shewed by very accurate experiments 

 that no animalcules are formed when the access 

 of air to the infusion is completely prevented, as 

 for example, when it is covered with a little oil, or 

 the vessel containing it is closely sealed ; and 

 he thence concluded that the germs of the ani- 

 malcules must exist in the atmosphere ; but the 

 supporters of the hypothesis consider them- 

 selves as entitled to hold that no production of 

 animalcules takes place in these circumstances, 

 merely because the exclusion of the air has the 

 effect of preventing that species of decomposi- 

 tion which they regard as necessary for the 

 formation of the Infusoria. 



It is stated by some experimenters that ani- 

 malcules are produced when the infusions are 

 exposed to hydrogen and nitrogen gases, or to 

 atmospheric air artificially prepared ; in which 

 it is held that there can be no living ova of 

 animalcules. Again, it appears from numerous 

 experiments, that when the infusions have been 

 exposed to a boiling temperature, which is ge- 

 nerally believed to have the effect of destroying 

 the life of all organized productions, the quan- 

 tity of animalcules formed is not diminished. 

 Some air, it has already been stated, must always 

 be present ; but so far as we are aware, the ex- 

 periments on this point have not been per- 

 formed in such a manner as to ascertain, whe- 

 ther or not, when an infusion is allowed to come 

 in contact with a considerable portion of con- 

 fined air, and the whole apparatus is exposed 

 to a temperature above that of boiling water, 

 the production of Infusoria may still take place; 

 and we are consequently obliged, in the absence 

 of more direct experiment, to have recourse to 

 analogical reasoning. 



The following considerations appear to us to 

 throw the balance of evidence in favour of the 

 spontaneous production of Infusoria, mould, 

 and the like. 



Firstly, those organic matters which are most 

 soluble in water, and at the same time most 

 prone to decomposition, give rise to the greatest 

 quantity of animalcules or cryptogamic plants. 



Secondly, the nature of the animalcule or 

 vegetable production bears a constant relation 

 to the state of the infusion, so that, in similar 

 circumstances, the same are always produced 

 without this being influenced by the atmo- 

 sphere. There seems also to be a certain pro- 

 gressive advance in the productive powers of 

 the infusion, for at the first the animalcules are 

 only of the smallest kinds or Monades, and after- 

 wards they become gradually larger and more 

 complicated in their structure ; after a time the 

 production ceases, although the materials are 

 by no means exhausted. When the quantity 

 of water is very small and the organic matter 

 abundant, the production is usually of a vege- 

 table nature; when there is much water, animal- 

 cules are more frequently produced. 



Thirdly, on the supposition that infusory ani- 

 malcules are developed from ova, it is neces- 



* Tracts on the Nature of Animals and Vegeta- 

 bles. Edin. 1799, (transl.) 



