68 NATURAL HISTORY 



The former refers to similarity of structure and the latter to similarity 

 of function. Thus the f orelimbs of a bat, bird, cat, and turtle are ail 

 homologous, while the wings of a bat or a bird are analogous to the 

 wings of a butterfly, but they are not homologous since they differ in 

 structure. 



A Classification of Plants and Animals 



As stated earlier, the appended scheme of classification is simply a 

 tool to be used by the student. Remember that a scheme of classi- 

 fication is not only the "who's who" of the plant and animal world 

 but it shows relationships as well, indicating what we know at present 

 in this field. Classification involves a knowledge of the occurrence, 

 distribution, development, and structure of the form studied, and 

 so is much more than simply applying a scientific name to an ani- 

 mal or plant. The use of scientific names cannot readily be avoided, 

 as will be realized from a study of these pages. If one really desires 

 to excel in biological work, he must set out cheerfully and with 

 determination to acquire an understanding of the use of these tools as 

 an indispensable aid to a comprehension of the interrelationship of 

 organisms to one another. 



In the first place it is hoped that diagrams which accompany this 

 classification are detailed enough to give the student some concept of 

 the more common or important kinds of representative organisms 

 occurring in each group. It is unfortunately impossible in these 

 drawings to represent the different animals according to scale. The 

 student hardly needs, however, to be reminded that whales and 

 protozoans should be interpreted as decidedly different in size. In 

 most cases, the classification will be carried only as far as the class, 

 although in a few groups, as with the Arthropoda and Tetrapoda, 

 it is necessary to go to the orders. In some instances attempts have 

 been made to simplify the classification in order to avoid unnecessary 

 scientific terminology. It should be added that the classification 

 here presented is only one of many that may be encountered in various 

 books, differing in details but agreeing in essential particulars. 



It is impossible to designate readily all of the characteristics which 

 are utilized in the separation of the larger plant and animal groups. 

 However, it is of importance to know (1) whether we are dealing with 

 a one- or many-celled form (uni- or multi-cellular) ; (2) the number 

 of germ layers present in the organism, diplohlastic — • two (ecto- 

 derm and endoderm) ; triplohlastic — three (ectoderm, endoderm, and 



