446 THE EVOLUTION OF LIFE. 



are well-marked species, and species so imperfectly marked 

 that some systematists regard them as varieties. Between 

 genera strong contrasts exist in many cases, and in other 

 cases contrasts so much less decided as to leave it doubt- 

 ful whether they imply generic distinctions. So, too, is it 

 with orders and classes: in some of which there have been 

 introduced sub-divisions, having no equivalents in others. 

 Even of the sub-kingdoms the same truth holds. The con- 

 trast between the Coelenterata and the Mollusca, is far less 

 than that between the Coelenterata and the Yertehrata. 



Now just this same indefiniteness of value, or incomplete- 

 ness of equivalence, is observable in those simple and com- 

 pound and re-compound groups which we see arising by 

 evolution. In every case the endeavour to arrange the 

 divergent products of evolution, is met by a difficulty like 

 that which would meet the endeavour to classify the branches 

 of a tree, into branches of the first, second, third, fourth, &c., 

 orders — the difficulty, namely, that branches of intermediate 

 degrees of composition exist. The illustration furnished by 

 languages will serve us once more. Some dialects of English 

 are but little contrasted ; others are strongly contrasted. The 

 alliances of the several Scandinavian tongues with one another 

 are different in degree. Dutch is much less distinct from 

 German than Swedish is; while between Danish and 

 Swedish there is so close a kinship that they might almost 

 be regarded as widely-divergent dialects. Similarly on com- 

 paring the larger divisions, Ave see that the various languages 

 of the Aryan stock have deviated from their original to very 

 unlike distances. The general conclusion is manifest. While 

 the kinds of human speech fall into groups, and sub-groups, 

 and sub-sub-groups; yet the groups are not equal to one 

 another in value, nor have the sub-groups equal values, nor 

 the sub-sub-groups. 



If, then, when classified, organisms fall into assemblages 

 such that those of the same grade are but indefinitely equiva- 

 lent; and if, where evolution is known to have taken place, 



