26 THE PROTOZOA 



The arbitrary dividing line between the Metazoa and the Protozoa 

 can be much more sharply drawn than that between animals and 

 plants. The Protozoa are usually defined as single-celled animals, 

 the Metazoa as many-celled ; but this definition is not strictly accurate, 

 for many forms of Protozoa live in aggregates, or colonies in which 

 specialization and division of labor have progressed to a considerable 

 degree ( Volvox, •Uroglena, Magosphcera, etc. ; Fig. 9). As a rule, 

 however, colonies do not form a distinct tissue of cells as in the blas- 

 tula stage of Metazoa, while a still stronger point is that they never 

 form a diblastic embryo. 1 



D. GENERATION DE NOVO 



Leeuwenhoek's discovery of the Protozoa had a marked effect 

 upon current thought, some speculative writers seeing in these minute 

 organisms the hypothetical units of organic structure, which, from the 

 time of Democritus to that of Descartes, had been a subject of 

 philosophical discussion. The rapid and incomprehensible increase 

 of Protozoa in standing water could apparently best be explained by 

 a theory of spontaneous generation ; Leeuwenhoek, nevertheless, was 

 convinced that their origin would be found in minute eggs or germs 

 which are carried through the air as dust, or brought from place to 

 place by birds, etc., thus showing his firm belief in Harvey's axiom, 

 ex ovo omnia. He was supported in this view by Joblot (1718), whose 

 experiments led him to the conclusion that the lower stratum of the 

 air is filled with the germs of various kinds of animalcula, while 

 Reaumur (1738) asserted that the dust of the air also contains dis- 

 ease germs, which are the cause of epidemics. These men were, 

 however, in the minority, and until the last fifty years only an occa- 

 sional observer opposed the theory of spontaneous generation, as 

 applied to these minute organisms. 



Even in Leeuwenhoek's time it was well known that dead organic 

 matter of any kind, when left exposed in water, gradually decom- 

 poses, while the water, at first clear, becomes murky, and minute 

 organisms of various kinds develop in it. Adopting the view that 

 higher organisms are composed of organic units, speculative writers 

 inferred that the small animals discovered by Leeuwenhoek were 

 the units which had again become freed from the aggregated condi- 

 tion. This is the key-note of Buffon's (1749) famous theory of gen- 

 eration, which, in one form or other, persisted well into the 19th 

 century. Briefly stated, Buffon believed that all organisms are 

 composed of an infinite number of organic particles. The In- 



J See Saville-Kent ('8i) for the obsolete theory that Sponges are colonial Protozoa. 



