38 HUMAN BIOLOGY 



from free nitrogen, carbon dioxide, phosphates, sulphates 

 and iron of their environment. 



The fact, however, that there are such ultramicroscopic 

 organisms is an indication that life is possible in exceedingly 

 minute, specially constituted, protein complexes, and we can, 

 accordingly, conceive that organisms of this type must have 

 been the very first to appear on the earth when its surface 

 conditions favored the evolution of hfe. The fact also that 

 such organisms multiply, indicates that they must increase 

 in size to permit this muItipHcation. Such increase in size 

 may lead to variations in volume and eventually to the 

 evolution of organisms of microscopic size, but with more 

 or less undifferentiated cytoplasm, from which developed 

 the ancestor of all the cells, animal and vegetable, of today. 



What was the origin of such a primal ultramicroscopic 

 organism? 



There have been advanced a number of theories and 

 hypotheses to account for the origin of life on the globe. 

 Only several of these are now worthy of attention, and a 

 brief reference to the character of such may be made here. 



There was first of all the theory of spontaneous generation. 

 This theory, first definitely advanced by Aristotle, but 

 held in a more or less inchoate form by earlier Greek 

 philosophers from Thales onward, postulated that living 

 organisms can arise spontaneously in media previously 

 free from all forms of life whatever. Lucretius expressed 

 this view, when he said that "the earth has rightfully 

 received the name of mother since all things are begotten of 

 it and many living creatures arise out of it, having been 

 generated by the rains and the warm mists formed by the 

 sun." Till the nineteenth century the behef in spontaneous 

 generation was almost universal and it was not an unnatural 

 result of observations made in an uncritical age. Some of 

 these were very superficial. One, based on the fact that 

 when a quantity of animal tissue, like beef muscle, is exposed 

 to the free air and sunshine at summer temperature larval 

 forms of flies and other insects appear in it in a few days, 

 was explained as spontaneous generation of these in putrefy- 

 ing flesh. This explanation was everywhere accepted until 

 Redi, in 1668, showed that when meat is placed in a wide- 



