THE DEGREES IN CLASSIFICATION. 



WE have discovered that the basis of classification is differ- 

 ence in structure.. We have arrived at a definition of a bird 

 which enables us to separate all winged, feathered, egg-lay- 

 ing, warm-blooded vertebrates into a class by themselves. We 

 call them a class because the line is so well marked between 

 them and all other living creatures. They form a group by 

 themselves as distinct as a class in a school a collection of 

 individuals that do the same work in the same way. 



But each of these classes may be sorted out still farther. 

 Let us take the class Birds. What do we notice first ? The 

 differences between them. We see that some are simple in 

 structure and some are complex, that they are of all degrees 

 of simplicity and complexity. So we divide them by their 

 rank into orders, just as classes in school are often divided 

 into sections according to their scholarship. An order is a 

 division by grade. Really an order means a row or line com- 

 posed of different objects of the same kind, especially a file 

 of soldiers. But we so often compare the lower orders and 

 the higher orders that it is perfectly correct to think of them 

 as divisions into grades. This grade is decided by men who 

 have studied every bone and muscle in the bird and who 

 know what each one means. They take no notice of the size 

 and color of the bird, but study its structure and decide 

 whether it is built on a simple or on a complex plan. 



The orders are divided into families. While it takes a very 

 wise man to decide what birds make up an order, it is not 



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