12 THE SMALLEST LIVING THINGS 



his own creation. Some specimens of these crude instruments 

 are in the British Museum. Yet with them Leeuwenhoek dis- 

 covered minute living things in rain water which had been left 

 exposed to air and sunlight. Some twenty-seven different kinds 

 of protozoa were described by him more or less adequately, 

 usually less, for today we are certain of only one of his types — 

 Vorticella (Fig. 5). The microscopic organisms he observed 

 were included by him under the comprehensive term "animal- 

 cula." Thousands of different types of microscopic forms have 

 since been added to Leeuwenhoek's collection and are today 

 classified as Bacteria, Algae, Protozoa, Rotifera, Worms, and 

 Crustacea — many of them representatives of higher animals. 



With this discovery in 1675 of the smallest living things 

 came a renewal of the belief in spontaneous generation, and with 

 regard to these minute organisms even the best scientific minds 

 wavered. It required two hundred years more to prove that 

 these microscopic organisms do not arise from lifeless matter 

 and that each one comes from an organism like itself. To the 

 biologist these minute living things are not only interesting in 

 themselves, but their study throws a flood of light on some fun- 

 damental problems of biology and it is with this in mind that the 

 present volume is written. 



Over a Hundred Thousand Species of Microscopic Forms 



There are upwards of 100,000 different species of these 

 microscopic forms, every species comprising representative living 

 organisms, each complete in itself. In some cases a particular 

 interest marks out one type from the rest, but for the most part 

 and from the point of view of the present volume, interest cen- 

 ters in the fact that in these minute organisms we have bits 

 of living matter which perform all of the vital activities per- 

 formed by the highest types of animals and plants. A complete 

 knowledge of any one of these living things would solve the 

 riddle of all life. The relative simplicity of the structures by 

 which these vital activities are performed make them relatively 

 simple to analyze. In the present treatment I shall make no 

 attempt to catalogue the microscopic forms, but I will try to 

 give an adequate idea of their general types of structure and of 

 the part they play in nature. 



