370 BIOLOGY 



organs owe their likeness to the fact that they have descended 

 from a common ancestor possessing such an organ. The bird, 

 the dog, and the monkey show homology in the wing, fore leg, 

 and arm, because they have descended from a common ancestor, 

 whose fore appendage possessed a certain series of bones and 

 muscles, and, therefore, all its descendants have, by inheritance, 

 retained these same bones and muscles. The differences between 

 the members in question have been brought about by the fact 

 that they were used for different purposes, and thus were slowly 

 modified in shape, although they still retained a fundamental 

 likeness in structure. 



CLASSIFICATION (TAXONOMY) 



Individuals. As we look upon nature to-day, we find only 

 individual organisms, each isolated from all others, and allied 

 only with its parents. But the most superficial examination 

 shows that some individuals have resemblances to each other, 

 while others are very unlike; and it is evident that organisms 

 can be arranged in groups showing more or less likeness to one 

 another. Such a grouping is called classification. The general 

 plan of such classification into groups is as follows: 



Species. When we find a large number of individuals re- 

 sembling one another so closely as to be practically identical, 

 we speak of them as belonging to a single species. For example, 

 the common dandelion, which is widely distributed over the 

 world, is made up of countless numbers of individuals; but they 

 are essentially alike, in root, in stem, in leaf, in flower. We 

 therefore speak of them all as constituting a single species, So 

 too is the horse a distinct species, and the ass another. To give 

 a definition of just what is meant by species is impossible, since 

 no one knows just what is meant, and the word perhaps does 

 not always have the same meaning. That the individuals of 

 a species are not always exactly alike is evident from facts 

 already mentioned concerning the great variations among dif- 

 ferent pigeons and dogs. Such great variations as those pre- 



