SECT. ii. INFUSORIA. 63 



cording to M. D'Orbigny the whole number of fossil 

 sponges known and described amount to thirty-six v 

 genera and 427 species, which is probably much belowj 

 the real number. 8 



CLASS IY. INTUSOBIA. 



The Infusoria, which form the second group of the 

 Protozoa, are microscopic animals of a higher grade 

 than any of the preceding creatures, although they go 

 through their whole lives as isolated single cells of 

 innumerable forms. They invariably appear in stag- 

 nant pools and infusions of animal and vegetable matter 

 when in a state of rapid decomposition. Every drop of 

 the green matter that mantles the surface of pools in 

 summer teems with the most minute and varied forms 

 of animal life. The species called Monas corpusculus 

 by the distinguished Professor Ehrenberg, has been 

 estimated to be ^oVo P ar ^ ^ a ^ ne ^ diameter. ( Of such 

 infusoria a single drop of water may contain 500,000,000 

 of individuals, a number equalling that of the whole 

 human species now existing upon the face of the earth. 

 But the varieties in size of these animalcules invisible 

 to the naked eye are not less than that which prevails 

 in almost any other natural class of animals. Prom / 

 the Monad to the Loxades or Amphileptus, which are i 

 the fourth and sixth part of a line in diameter, the j 

 difference in size is greater than between a mouse and J 

 an elephant ; within such narrow bounds might our 

 ideas of the range in animal life be limited if the sphere 

 of our observation was not augmented by artificial aid.' 9 



This singular race of beings has given rise to the 

 erroneous hypothesis of equivocal or spontaneous gene- 

 ration, that is to say, the production of living ani- 

 malcules by a chemical or even fortuitous combination 



8 ' Palaeontology,' by Professor Owen. 9 Prof. Owen. 



