DISTINCTIVE CHARACTERS OF THE PROTOZOA 



understood. The term Protista thus unites under a single 

 systematic category the vast assemblage of simple and primitive 

 living beings from which the animal and vegetable kingdoms have 

 taken origin, and have developed, by a continuous process of 

 natural evolution, in different directions in adaptation to divergent 

 modes of life. 



The conception of a Protistan kingdom separate from the animal and 

 vegetable kingdoms is open to the objection that it contains organisms 

 which are indubitably of animal or vegetable 

 nature respectively. The relations of the 

 Protista to other living things may be repre- 

 sented graphically by the accompanying dia- 

 gram (Fig. 1), wnere the circle represents the 

 Protista, the two triangles the animal and 

 vegetable kingdoms respectively. It is seen 

 that the separation of the Protista as a 

 systematic unity cuts across the ascending 

 series of evolution ; to express it figuratively, 

 it is a transverse cleavage of the phylogenetic 

 " tree." A truly natural classification of living 

 things, however, is one which expresses their 

 genetic affinities and follows their pedigrees 

 and lines of descent ; it should represent a 

 vertical cleavage of the ancestral tree. Judged 

 by this standard, the kingdom of the Protista 

 can only be regarded as a convenient makeshift 

 or compromise, rather than as a solution of a 

 difficult problem that, namely, of giving a 

 natural classification of the most primitive 

 forms of life. 



FIG. 1. Graphic representa- 

 tion of the relation of the 

 animal and vegetable king- 

 doms to the kingdom of the 

 Protista (Protislenreich). 

 The Protozoa are represented 

 by the portion of the triangle 

 representing the animal 

 kingdom which lies within 

 the circle representing the 

 Protista. 



Whether the kingdom Protista be 

 accepted or not as a natural and valid 

 division of living beings, it is imperative 

 to subdivide it further, not only on 

 account of its vast extent and unwieldy 

 size, but also because it comprises organisms very diverse in nature, 

 requiring for their study the application of methods of technique 

 and investigation often entirely different in kind. Hence in actual 

 practice the Protista are partitioned among at least three different 

 classes of scientific workers zoologists, botanists, and bacteri- 

 ologists each studying them by special methods and to some extent 

 from different points of view. 



It is necessary, therefore, to consider from a general standpoint 

 the principal types of organization comprised in the kingdom 

 Protista, and we can recognize at the outset two chief grades of 

 structure, bearing in mind always that transitional forms between 

 them must exist, or at least must have existed. 



In the first grade, which is represented by the Bacteria and allied 

 groups of organisms, a type of organization is found which is 

 probably the more primitive, though by many regarded as the 



