308 THE INDUCTIONS OF BIOLOGY. 



are distinguished from each other, either by modifications in 

 the structures of fundamental parts, or by the presence or 

 absence of subsidiary parts, or by both. Fishes and Am- 

 phibia are unlike higher vertebrates in possessing branchiae ; 

 either throughout life or early in life. And every higher 

 vertebrate, besides having lungs, is characterized by having, 

 during development, an amnion and an allantois. Mammals, 

 again, are marked off from Birds and Bep tiles by the 

 presence of mammae, as well as by the form of the occipital 

 condyles. Among Mammals, the next division is based on 

 the presence or absence of a placenta. And divisions of the 

 Placentalia are mainly determined by the characters of the 

 organs of external action. 



Thus, without multiplying illustrations and without de- 

 scending to genera and species, we see that, speaking gener- 

 ally, the successively smaller groups, are distinguished from 

 one another by traits of successively less importance, physio- 

 logically considered. The attributes possessed in common 

 by the largest assemblages of organisms, are few in number 

 but all-essential in kind — affect fundamentally the most vital 

 actions. Each secondary assemblage, included in one of the 

 primary assemblages, is characterized by further common 

 attributes that influence the functions less profoundly. And 

 so on with each lower grade of assemblage. 



§ 103. What interpretation is to be put on these truths of 

 classification ? We find that organic forms admit of an 

 arrangement everywhere expressive of the fact, that along 

 with certain attributes, certain other attributes, which are 

 not directly connected with them, always exist. How are 

 we to account for this fact ? And how are we to account for 

 the fact that the attributes possessed in common by the 

 largest assemblages of forms, are the most vitally-important 

 attributes ? 



No one can believe that combinations of this kind may 

 have arisen fortuitously. Or if any one believes this, it is 



