CHAPTER VI 



THE BORDERLINE BETWEEN PLANTS AND 



ANIMALS* 



RARELY is the unsightliness of a scum-covered pool due solely 

 to animal life. On the contrary, it is usually brought about 

 by the rich growth of filamentous plants, the blue-green algae 

 or the grass-green algae.''' These, although beautiful and inter- 

 esting under the microscope, scarcely come within the limits of 

 the present volume. Living in the water among these plants, 

 however, one always finds a great variety of unicellular animals 

 (protozoa), and unicellular plants, some grass-green in color 

 {e.g., desmids), some yellow {e.g., diatoms), and many chloro- 

 phyll-bearing motile forms. 



When Leeuwenhoek, in 1675, revealed the world of micro- 

 scopic forms, he grouped them, plants and animals alike, under 

 the general name animalcula.K Later observers, using better 

 microscopes and aided by an increased knowledge of living things, 

 gradually placed the forms which were definitely plant-like in the 

 field of botany and those which were definitely animal-like in 

 that of zoology. Such segregation obviously depends upon our 

 definitions of "plant" and "animal," and is not based upon any 

 clear-cut demarcation in animate forms. As a result it has been 

 impossible to word a definition which will enable us unfailingly 

 and without confusion to place among plants or among animals 

 those unicellular forms which lie on the boundary line between the 

 fields of botany and of zoology. The German biologist, Ernst 

 Heinrich Haeckel, in 1866, sought to avoid the difficulty by 

 creating the term protista to designate all unicellular plants and 

 unicellular animals, a term from which we get protistology for 

 the science dealing with unicellular life. 



* See "The Plant World," pages 3-9, and "The Animal World," pages 38-40, 

 both in this Series. 



t See "The Plant World," pages 13-17, in this Series. 

 t Singular, Animalculum. 



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