THE LIVING MACHINE BUILDING FACTORS. 165 



likeness to each other while others are much 

 more unlike. The grouping of animals and plants 

 into orders, genera, and species is dependent 

 upon this relationship. If two forms are alike 

 in everything except some slight detail, they are 

 commonly placed in the same genus but in differ- 

 ent species, while if they show a greater unlike- 

 ness they may be placed in separate genera. By 

 thus grouping together forms according to their 

 resemblance, the animal and vegetable kingdoms 

 are classified into groups subordinate to groups. 

 The principle of relationship, i.e., fundamental 

 similarity of structure, runs through the whole 

 animal and vegetable kingdom. Even the ani- 

 mals most unlike each other show certain points 

 of similarity which indicates a relationship, al- 

 though of course a distant one. 



The fact of such a relationship is too patent 

 to demand more words, but its significance 

 needs to be pointed out. When we speak of 

 relationship among men, we always mean his- 

 torical connection. Two brothers are closely 

 related because they have sprung from common 

 parents, while two cousins are less closely re- 

 lated because their common point of origin was 

 farther back in time. More widely we speak 

 of the relationship of the Indo-European races, 

 meaning thereby that back in the history of 

 man these races had a common point of origin. 

 We never speak of any real relation of objects 

 unless thereby we mean to imply historical con- 

 nection. We are therefore justified in inter- 

 preting the manifest relationships of organisms 

 as pointing to history. Particularly are we 



