ROLL CALL 57 



However, species have otlicr characteristics besides oxtornal or 

 morphological similarities. They breed true, that is, cats produce 

 cats, and dogs produce dogs. Usually diflferent species cannot be 

 crossed. There are exceptions, for sometimes one species crossed with 

 another may yield a sterile hybrid. Thus a horse crossed with an ass 

 produces a mule. But on the whole the preceding statement holds 

 true. 



Two criteria have been used in classifying organisms, first, struc- 

 tural differences, or appearance, which really means comparative mor- 

 phology, checked physiologically and genetically by the cross-breeding 

 of species, and second, the approach through a study of the early 

 development and the life cycle, emhnjology, and the distribution 

 of the organism, ecology. The latter leads to a consideration of 

 varieties, subspecies, and races, which through mtergradations often 

 complicate the problem of determining species. 



Such a study may be made either more complicated or facilitated 

 according to whether a so-called natural classification or artificial 

 classification is utilized. Thus bats and birds might be artificially 

 classified together, simply because they both fly, just as whales and 

 fishes are placed together by the ignorant, because both inhabit 

 the water. A careful study of the anatomy and development of these 

 animals would indicate that if one is trying to show relationships, 

 which is what a classification should do, bats and whales would both 

 have to be put in the mammalian group. 



Determination of the type of symmetry present is useful in clas- 

 sification. Some organisms possess a universal symmetry, as the 

 protozoan Volvox. In such cases the organism is divided into equal 

 halves by any plane that passes through the center. Starfish and 

 hydra, on the other hand, are well-known examples of radial sytn- 

 metry. In such forms there is a single axis, as may be seen in a 

 cylinder, and a number of planes through such an axis would di\-id(^ 

 the organism into symmetrical halves. Most of the more highly 

 developed forms possess bilateral symmetry, which is characterized 

 by similar halves on either side of a main axis. Other secondarv' 

 planes occur in bilaterally symmetrical animals, resulting in anterior- 

 posterior, and in dorso-ventral differentiation. Sometimes segmenta- 

 tion, or metamerism, is apparent, as in the case of the earthworm and 

 many of the Arthropods. 



If one attempts a classification that is based primarily upon struc- 

 ture, it is necessary to differentiate between homology and analogy. 



