THE ARGUMENTS FROM CLASSIFICATION. 447 



there have arisen assemblages between which the equivalence 

 is similarly indefinite; there is additional reason for in- 

 ferring that organisms are products of evolution. 



§ 126. A fact of much significance remains. If groups of 

 organic forms have arisen by divergence and re-divergence; 

 and if, while the groups have been developing from simple 

 groups into compound groups, each group and sub-group has 

 been giving origin to more complex forms of its own type; 

 then it is inferable that there once existed greater structural 

 likenesses between the members of allied groups than exists 

 now. This, speaking generally, proves to be so. 



Between the sub-kingdoms the gaps are extremely wide; 

 but such distant kinships as may be discerned, bear out anti- 

 cipation. Thus in the formation of the germinal layers there 

 is a general agreement among them; and there is a further 

 agreement among sundry of them in the formation of a 

 gastrula. This simplest and earliest likeness, significant of 

 primitive kinship, is in most cases soon obscured by divergent 

 modes of development; but sundry sub-kingdoms continue 

 to show relationships by the likenesses of their larval forms; 

 as we see in the trochophores of the Polyzoa, Annelida, and 

 Mollusca — sub-kingdoms the members of which by their later 

 structural changes are rendered widely unlike. 



More decided approximations exist between the lower 

 members of classes. In tracing dowTi the Crustacea and the 

 Arachnida from their more complex to their simpler forms, 

 zoologists meet with difficulties: respecting some of these 

 simpler forms, it becomes a question which class they belong 

 to. The Lepidosiren, about which there have been disputes 

 whether it is a fish or an amphibian, is inferior, in the organi- 

 zation of its skeleton, to the great majority of both fishes and 

 amphibia. Widely as they diifer from them, the lower mam- 

 mals have some characters in common with birds, which the 

 higher mammals do not possess. 



Now since this kind of relationship of groups is not ao- 



