NATURE OF REPRODUCTION. 677 



in 1838 1 described more than 700 different kinds. They are generally 

 provided with cilia attached to the exterior of their bodies, and are, for 

 the most part, in constant and rapid motion in the fluid which they 

 inhabit. 



In consequence of the numerous different forms of the infusoria, their 

 frequent changeability of figure, and their want of resemblance to any 

 previously known class of animal organisms, they were thought, by 

 some of the earlier observers, to have no regular mode of generation, 

 but to arise indiscriminately from the organic materials of the infusion ; 

 the particular form which they might assume being determined by the 

 special conditions of each case. Their inevitable appearance in organic 

 infusions, at all ordinary temperatures and exposures, contributed to 

 sustain this belief. The substance of the infusion might be previously 

 baked or boiled ; the water in which it was infused might be distilled, and 

 thus freed from all organic contamination ; and yet the infusoria would 

 make their appearance at the usual time and in the usual abundance, 

 provided only that the infusion were exposed to moderate warmth and 

 to the access of atmospheric air. But these conditions are essential to 

 maintaining the life of all organized creatures, from whatever source 

 they may come, and are not, therefore, more necessary to the infusoria 

 than to others. 



Therefore the infusoria must either have been spontaneously gene- 

 rated from the materials of the infusion, or else they must have been 

 produced from germs introduced from the atmosphere. In the latter 

 case these germs must be wafted about, in a comparatively dry state 

 and in an inactive condition, by the atmospheric currents, to resume 

 their activity and development when brought in contact with sufficient 

 moisture and with the organic material requisite for their nutrition. 



The researches relating to this question continued with the most extra- 

 ordinary persistence, and with various interruptions and revivals, from 

 1775, when they were carried on by Needham and Spallanzani, through- 

 out the greater part of the present century, in the hands of Cuvier, 

 Schultze, Helmholtz, Milne-Edwards, Longet, Pouchet, Pasteur, Wy- 

 man, and Bastian. The main object of investigation was to discover 

 whether, if all previous living germs were destroyed by heat, and the 

 access of others prevented by hermetically sealing the vessels, or 

 thoroughly purifying the air which was introduced, infusorial life 

 would, under such circumstances, be developed. 



The general result of these experiments was that such precautions 

 diminished and often entirely prevented the production of infusoria. 

 Spallanzani 2 had already shown in 1776 that organic infusions in her- 

 metically sealed glass flasks, if boiled for two minutes, failed to produce 

 any of the larger and more highly organized animalcules; and that 

 boiling for three-quarters of an hour prevented the appearance of the 

 more minute and simpler kinds. 



1 Die Inftisionsthierchen als vollkommene Organismen. Leipzig, 1838. 



2 Opuscoli de Fisica animale e vegetabile. Modena. 1776, vol. i. p. 10. 



