4 THE PROTOZOA 



Protozoa, then there must have existed an unbroken series of 

 transitions between these two types of living beings. Hence, as 

 in all attempts to classify living beings, sharp verbal distinctions 

 between Protozoa and Metazoa are rendered possible only through 

 the extinction of intermediate forms, or by ignoring such forms if 

 known to exist. It is expedient rather to recognize distinct types 

 of organization characteristic of the Protozoa and the Metazoa 

 respectively, and to compare and contrast them, than to attempt 

 to limit these groups by precise definitions. 



2. " Animals." This part of the definition raises more difficulties 

 than their cellular nature. In the higher forms of life the distinc- 

 tion between animals and plants is an obvious and natural one ; it 

 is by no means so in the lower organisms. In the ranks of the 

 simplest living creatures, those of animal nature are not marked 

 off by any sharply defined structural or other features from those of 

 vegetable nature, and cannot be separated from them in any scheme 

 of classification which claims to be founded upon, or to express, the 

 true natural affinities and relationships of the objects dealt with. 

 As will be explained more fully in the next and subsequent chapters , 

 the distinction between animal and vegetable is, at its first appear- 

 ance, nothing but a difference in the mode in which the organisms 

 obtain their living. Forms that are obviously closely allied in all 

 their characters may differ in this respect, and in some cases even 

 one and the same species may nourish itself at one time as a plant, 

 at another as an animal, according to circumstances. In short, the 

 difference between plant and animal is primarily a distinction based 

 upon habits and modes of life, and, like all such distinctions, does 

 not furnish characters that can be utilized for systematic classifica- 

 tion until the mode of life has continued so long, and the habit has 

 become so engrained, as to leave an impress upon the entire 

 structural characteristics of the organism. 



The Protozoa cannot therefore be defined strictly and con- 

 sistently as organisms of animal nature, for, though the vast majority 

 of them certainly exhibit animal characteristics, it is impossible to 

 exclude from the group many which live temporarily or permanently 

 after the manner distinctive of the vegetable kingdom, and which 

 are plants, to all intents and purposes, leading on in an unbroken 

 series to the simplest algae. 



For this reason it has been proposed to unite all the simplest and 

 most primitive forms of life in one " kingdom " under the title 

 Protista (Protistenreich, Haeckel), irrespective of their habit of life 

 and metabolism, whether animal or vegetable. The kingdom 

 Protista is then to be considered as equivalent in systematic value 

 to the animal and vegetable kingdoms, which in their turn are 

 restricted in their application to true animals and plants as ordinarily 



