THE ORIGIN OF LIFE 



sedentary, and the individual cells are surrounded by a rigid cellulose 

 wall, while such rigid walls are generally lacking on animal cells, and the 

 animal moves about freely in its environment. 



As measured against such a group of criteria, Euglena and its allies are 

 difficult to place. Euglena itself is supplied with an abundance of chloro- 

 phyll, yet it can be raised on completely inorganic media only with diffi- 

 culty. Traces of amino acids or peptones facilitate culture. Nonetheless, 

 there is no evidence that Euglena ever ingests other organisms; it seems 

 more probable that its normal nutrition is predominantly holophytic (by 

 photosynthesis) with a supplement being obtained saprozoically. Some 

 other flagellates, however, are completely holophytic. Others lack chloro- 

 phyll, and these of course cannot be autotrophic. Some of them are en- 

 tirely saprozoic, but some ingest other organisms in typical protozoan 

 fashion and so may be said to be holozoic. Thus the whole range of nutri- 

 tional possibilities occurs within a single group, and opposite extremes 

 may occur within a single genus. Stored food in plants is ordinarily starch, 

 while in animals it is either glycogen (similar to starch) or fat. In the 

 euglenoids, it is paramylum, a carbohydrate different from both starch 

 and glycogen. The embryological criteria of external or internal organ 

 formation and presence or absence of a continuously growing meristem 

 obviously have no applicability to unicellular organisms. As might be 

 expected of an animal, Euglena moves freely in its environment, but its 

 near relatives include sedentary forms. Finally, while Euglenoids lack a 

 cellulose cell wall, they do have a pellicle which in some species is quite 

 rigid. 



One result of this mixture of plant and animal characteristics among 

 the unicellular, flagellate organisms is a much confused area in taxonomy. 

 Some biologists have treated the whole array as plants, a procedure which 

 makes it necessary to treat as plants such organisms as the trypanosomes, 

 blood parasites of vertebrates which do not show any plant-like charac- 

 teristics. Others have tried to designate some forms as plants and others 

 as animals on the basis of the above criteria or similar ones. This involves 

 one in the absurd situation of assigning different members of the same 

 genus to different kingdoms in some instances. Often the whole series of 

 algae and Protozoa have been lumped together as a single kingdom 

 Protista, with only the Metazoa left in the Animal Kingdom and only the 

 vascular plants and the bryophytes left in the Plant Kingdom. This is 

 again unsatisfactory because the higher algae are obviously much more 

 closely related to the vascular plants than to many of the Protozoa, and 

 conversely the animal nature of many of the Protozoa, such as the ciliates, 

 is not open to doubt. 



Copeland has made a very radical suggestion, but witli much good 

 reason. He suggests that the living world be divided into four kingdoms. 

 Kingdom I, the Mychota, would include the bacteria, and the blue-green 

 algae, that is, all organisms in which the nuclear-cytoplasmic differentia- 

 tion is not complete. Kingdom II, the Protoctista, would include most of 

 the algal phyla, the fungi, and Protozoa. Thus the kingdom Protoctista 

 includes all of those primitive organisms from which higher plants and 



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