Birds. loi 



collateral characters are given, but these sufficiently distinguish 

 birds from reptiles on the one hand and mammals on the other. 

 Perhaps the majority of modern zoologists accept Huxley's arrange- 

 ment of birds and reptilesas off-shoots from one group — the Sai/rop- 

 sida. 



As already stated, stniiturc is the only basis of modern classi- 

 fications. 



Formerly classifications were based on habits, food, distribu- 

 bution, etc; but these have all been found to be unreliable and 

 subject to change regardless of structure. 



Various similes have been used to illustrate ideas of relation- 

 ship in structure of the different forms of life. 



One of the oldest of these is to liken species of animals to 

 the links of a cliain and arrange them in a linear series, with 

 gaps representing extinct or undiscovered species. This would 

 be equivalent to the attempt to place all mankind in a con- 

 tinous row, in the order of their relationship to each other; and 

 you may readily see, that, if we attempted this we should very soon 

 find hvo persons who were of equal relationship to a third *and 

 hence our linear series would become bi linear, then tri-linear and 

 so on, so that the resemblance to a chain would disappear. 



A second simile, is that of a tree, with few main divisions, 

 more large branches and numerous twigs; this is, I think best 

 suited to the illustration of all Biological problems. If we compare 

 life then, with the trunk of our tree, (Fig. i) we have two great 

 divisions of organisms, which act as vehicles or agents for its 

 expression — viz : Vegetable and AAimal, — these divisions called in 

 systematic Biology "Kingdoms." 



Leaving the study of the vegetable part of our tree to the 

 botanist, and tracing out the Animal Kingdom, we find that all 

 animals might be conveniently placed in five great groups, occupy- 

 ing five great branches of our tree, so to speak. These groups, 

 called 6"// (^-A7;/^^''(/(W/.s- are : (see diagram, Fig. i.) 



1. The Protozoa (first animals) or simplest animals, (Infusoria 



etc.) 



2. The Radiata or radiated animals, (Star-fishes, sea-ur- 



chins.) 



3. The Articulata or jointed animals, (Insects, lobsters, etc.) 



4. The MoUusca or soft-bodied animals, (Snails, oysters, 



mussels, etc.) 

 q. The Vertebrata or back-boned animals. 



