74 UNDERWATER GUIDE TO MARINE LIFE 



fossil record is far from complete and since the interpretation of it is subject 

 to human frailty and differences of opinion, there are several systems of classifi- 

 cation which involve duplications and overlaps of the same type that occur in 

 common names. Are the triggerfishes, trunkfishes, and puffers three separate sub- 

 orders, or are they three parts of one suborder? Opinions differ and names are 

 invented to express the differences. Luckily, there is little difference of opinion 

 regarding families, and they can be given the strongest reliance. This is due to 

 the fact that the families of animals and plants form fairly distinct groups. For 

 some reason, the transitional species that unite families are short-lived and 

 usually do not stay around long enough to give scientists a chance to examine 

 them or their fossil remains. This is probably because families share the char- 

 acteristics of an adaptive type which is established quickly in evolution, so 

 quickly that there are few intermediates. Basically, the classificatory system of 

 living things is built as follows: 



Kingdom: only two in number— animal and plant. 



Phylum: the largest subdivision, e.g., Chordata, including protovertebrates 

 and vertebrates. 



Class: a major phylum subdivision, e.g., Osteichthyes, including all bony fishes. 



Order: a prominent assemblage of families, e.g., Apodes, including all true eels. 



Family: the basic "adaptive type" grouping, e.g., Muraenidae, including all 

 moray eels. 



Genus: a distinct morphological tvpe; e.g., Gymnothorax, including an as- 

 semblage of very similar species. 



S'pecies: the unit of interbreeding individuals, e.g., Gymnothorax moringa, 

 the spotted moray eel. 



It will be noticed that there are seven subdivisions in this svstem. As we 

 progress down from kingdom to species, the evolutionary relationship between 

 comparable groups becomes closer and closer Cfig. 17^. Most often, however, 

 seven subdivisions are not enough to express the numerous branchings of the 

 evolutionary tree, so the prefixes "sub," "super," or "infra" mav be added to the 

 seven primary divisions or new categories such as cohort or tribe may be added. 

 The basic system between class and genus may thus be expanded: 



Cohort 



Superorder 



Order 



Suborder 



Infraorder 



Superfamily 



Family 



Subfamily 



Tribe 



Just as evolution is complex and increases in complexity as we learn more 

 about it, so the classificatory system increases in complexity in an attempt to 

 reflect evolutionary patterns. There is some effort to stabilize this system. This 



