98 



CLASSIFICATION, PHYLOGENY, AND EVOLUTION: 



NAME CHANGES 



In general, scientific names are quite stable. 

 However, there are circumstances under which 

 scientific names can be changed. The two main 

 sources of redesignations are taxonomic revisions and 

 the law of priority. In part, the law of priority states 

 that the first name given a species shall be its scien- 

 tific name. 



Changes due to taxonomic revision generally re- 

 flect scientific progress. They are based upon new 

 interpretation of biological relationships as a result 

 of study of one or more species. This can cause one 

 or more of three possible name changes: (1) A species 

 must be transferred from one genus to another, so the 

 generic name becomes difTerent. (2) It is discovered 

 that what had been considered two species is a single 

 species, so one specific name must be dropped, or 

 synonomized, and the first given name be established 

 for the species as a whole (law of priority). (3) 

 What was thought to be a single species is found to be 

 two or more species, in which case new specific names 

 are given to each of the new species. All of these 

 examples are "scientific changes of names. " 



Reassignments due to the law of priority alone are 

 the common example of "nomenclatorial changes in 

 names." These changes often are criticized be- 

 cause they do not stem from new knowledge or 

 scientific progress. The technicalities in relation to 

 the law of priority and to other bases for nomenclato- 

 rial changes in names are extremely complex. How- 

 ever, a simple example of a replacement according to 

 the law of priority would involve a generic and/or 

 specific name's being superseded because a properly 

 and earlier applied name is found. In spite of the 

 fact that these alterations do cause difficulty and 

 exasperation, they are an aspect of the very thing 

 that yields the great stability characterizing most 

 scientific names. 



PHYLOGENY 



The evolution of monerans, protistans, plants, and 

 animals followed, in each case, certain major "steps" 

 in the development of structural complexity that now 

 are indicated by the fossil record and development of 

 living creatures. 



The evidence of these steps is the basis in this 

 book for the grouping of phyla into four kingdoms. 

 The grouping to be presented in the chapters that 



follow does not seem to follow any authority on 

 classification exactly, but is closest to the over-all 

 proposal of Weisz. In spite of its "unusual" nature, 

 the classification used here should not cause dif- 

 ficulty in the use of other books. The animal phyla 

 follow the widely used classification of Hyman, 

 with the exception that Phylum Protozoa is not in- 

 cluded. The phyla here included under Kingdom 

 Protista, Subkingdom Protozoa are often considered 

 subphyla or classes by other classifiers. 



Phyla other than those currently placed in the 

 Animal Kingdom and Protozoa are identical with 

 those proposed by most botanists. 



MONERA, PROTISTA, AND PLANT 

 COMPLEXITY 



According to our assumptions the earliest steps in 

 organism evolution were associated with the increas- 

 ing complexity of protoplasm (Figure 7.1). First was 

 the diflferentiation of protoplasm itself, the Monera 

 stage. Second was a further tendency toward the or- 

 ganization of definite cell-like structure, the earliest 

 Protista stage. This segregation of nucleus, cytoplasm, 

 and cell membrane created the component parts of 

 what later became true cells. A cell-like body that 

 is an entire organism is said to have acellular or uni- 

 cellular or^anizauon. The third step was formation of 

 complex cell-like structure, including a distinct cell 

 wall, plastids, and many different ctyoplasmic pig- 

 ments associated with the plastids; this, the later 

 Protista stage, is still represented by living chlorophyll- 

 bearing, flagellate protists. In the fourth step, com- 

 plexity continued as single acellular organisms gave 

 rise to colonial ones, the colonial Protista stage. Finally, 

 colonial protists, probably chlorophyll-bearing 

 flagellates, gave rise to multicellular life, hence true 

 cellular organization and the Plant stage: possibly 

 even later, other protists, perhaps both flagellates 

 and ciliates, evolved into other multicellular life, the 

 animals. The great importance of multicellularity was 

 that it became the primary impetus to further in- 

 crease in size and complexity of life. 



REPRODUCTIVE MECHANISMS 



The first life very likely reproduced asexually by 

 fission or perhaps a simpler mechanism; However, it 

 is thought that prior to multicellularity, sexual re- 

 production was de«ved from asexual means. Then 



