20 ILLUSTRATION OF NATURAL GROUPS. 



into Families, each family into Genera, and each genus 

 into Species composed of individuals essentially alike. 



These groups must not be regarded as mere artificial 

 contrivances ; on the contrary, they rest on a natural basis. 

 And, according to Agassiz : 



Branches are characterized by plan of structure. 



Classes, by the manner in which the plan is executed, as far as ways 

 and means are concerned. 



Orders, by the complication of structure. 



Families, by form, as determined by structure. 



Genera, by details of execution in special parts. 



Species, by the relation of individuals to one another, as well as by 

 the proportion of their parts, their ornamentation, etc. 



That is, according to this view, certain characters deter- 

 mine the Branch, certain others the Class, others the Or- 

 der, others the Family, others the Genus, and others still 

 the Species. 



These principles of classification, however, are not uni- 

 versally recognized and fully applied ; but most, or many 

 naturalists designate at least the subordinate groups by a 

 combination of characteristics more or less different from 

 one another in their nature. 



But it must be stated here that all scientific classification 

 of animals is based essentially upon structure upon struc- 

 ture, and upon form as determined by structure. This is 

 strictly true unless we find an exception in the fact that, 

 in deciding upon species, we must also consider size and 

 color. 



As a simple illustration of the natural groups of animals 

 enumerated above, it may be stated that : - 



All animals which have a back-bone constitute one 

 Branch called the Vertebrates; 



All of the back-boned animals which are covered with 

 feathers, and organized for flight, constitute one Class, called 

 the Birds ; 



