THE LIVING MACHINE BUILDING FACTORS. 143 



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 relation- 

 ship among men we always mean historical con- 

 nection. Two brothers are closely related be- 

 cause they have sprung from common parents, 

 while two cousins are less closely related 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 connection. We are therefore 

 justified in interpreting the manifest relationships 

 of organisms as pointing to history. Particularly 

 are we justified in this conclusion when we find 

 that the relationships which we draw between the 

 types of life now in existence run parallel to the 

 history of these types as revealed to us by fossils 

 and at the same time disclosed by the study of 

 embryology. 



This subject of comparative anatomy includes 

 a consideration of what is called homology, and 

 perhaps a concrete example may be instructive 

 both in illustration and as suggesting the course 

 which nature adopts in constructing her machines. 



