But is there a transition from Radiates to Mollusks, or from Articulates to Vertebrates, or from any one of these divisions into any other?  Let us first consider the classes as they stand within their divisions.  We have seen that there are three classes of Radiates,—­Polyps, Acalephs, and Echinoderms; three classes of Mollusks,—­Acephala, Gasteropoda, and Cephalopoda; three classes of Articulates,—­Worms, Crustacea, and Insects; and, according to the usually accepted classification, four classes of Vertebrates,—­Fishes, Reptiles, Birds, and Mammalia.  If there is indeed a transition between all these classes, it must become clear to us, when we have accurately interpreted their relative standing.  Taking first the lowest branch, how do the classes stand within the limits of the type of Radiates?  I think I have said enough of these different classes to show that Polyps as a whole are inferior to Acalephs as a whole, and that Acalephs as a whole are inferior to Echinoderms as a whole.  But if they are linked together as a connected series, then the lowest Acaleph should stand next in structure above the highest Polyp, and the lowest Echinoderm next above the highest Acaleph.  So far from this being the case, there are, on the contrary, many Acalephs which, in their specialization, are unquestionably lower in the scale of life than some Polyps, while there are some Echinoderms lower in the same sense than many Acalephs.  This remark applies equally to the classes within the other types; they stand, as an average, relatively to each other, lower and higher, but considered in their diversified specification, there are some members of the higher classes that are inferior in organization to some members of the lower classes.  The same is true of the great divisions as compared with each other.  Instead of the highest Radiates being always lower in organization than the lowest Mollusks, there are many Star-Fishes and Sea-Urchins higher in organization than some Mollusks; and so when we pass from this branch to the Articulates, if we assume for the moment, as some naturalists believe, that the Mollusks are the inferior type, the Cuttle-Fishes are certainly very superior animals to most of the Worms; and passing from Articulates to Vertebrates, not only are there Insects of a more complex organization than the lowest Fishes, but we bring together two kinds of animals so remote from each other in structure that the wildest imagination can scarcely fancy a transition between them.  A comparison may make my meaning clearer as to the relative standing of these groups.  The Epic Poem is a higher order of composition than the Song,—­yet we may have an Epic Poem which, from its inferior mode of execution, stands lower than a Song that is perfect of its kind.  So the plan of certain branches is more comprehensive and includes higher possibilities than that of others, while at the same time there may be species in which the higher plan is executed in so simple a manner