CHAPTER VII 



EVOLUTIONARY DEVELOPMENT: RELATIONSHIPS, DERIVA- 

 TION, DEVELOPMENT WITHIN TLIE GROUP, STRUCTURAL 

 EVOLUTION, NUTRITION AND EVOLUTION, 

 RELATIONS TO THE METAZOA 



The phylogeny of the Protozoa is a subject obscured by couditious inherent 

 in the organisms themselves, namely, the unstable and evanescent character of 

 their substance which, with many of the groups, requires but one minute or 

 less to disapi^ear absolutely without leaving the faintest trace behind. This is 

 especially true of the Flagellata. "While many of the flagellates have acquired 

 more resistant structures, such as the theea of the dinoflagellates, yet these 

 represent the terminations of the lines of evolution, and their preservation 

 throws no light on the earlier stages of their development or their interrelations. 

 The Flagellata also probably represent the most prunitive class, with the pos- 

 sible exception of the lower bacteria, of which we have any adequate knowledge 

 today, and stand at the base of the phylogenetic trees of both the plant and 

 animal world. Thus the length of time during which they have been under- 

 going evolutionary development far surpasses that of any other group of li^dng 

 organisms of the present day, and at the same time provides for a wider range 

 of differentiation and the more complete elimination of -many connecting links. 

 This has resulted in the production of a group of organisms exhibiting, within 

 the compass of a single, minute cell, a wider range of differentiation and of 

 structural features than may lie met with in any phylum or even groups of 

 phyla above the Protista. 



Another factor tending to obscure our view of the evolution of these 

 organisms, but one which is daily giving way before the constant advance of 

 new discoveries in this field, is the lack of knowledge of the full life history of 

 the Protozoa. The older conception of a simplicity of life history for these 

 forms is gradually being replaced by a more adequate concei^tion of what may 

 be the results on these plastic organisms of a long period of evolution in a 

 changing environment. In line with this comes a change also in the older idea 

 that these are the most simple forms of life. Indeed, the complexity whicli 

 we now find resident in a single cell, often containing neuromotor organelles 

 or an ocellus in addition to the usual complement of organelles for the purposes 

 of ingestion and digestion of food, secretion, excretion, and other life processes, 

 postulates a relatively higher stage of evolution than that reached by some of 

 the Metazoa. On the other hand, many (if the flagellates especially are among 

 the simplest of known organisms both morphologically and in their life histoiy, 

 as far at least as these are known, excluding of coTii'se the bacteria. 



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