if amphioxus is a vertebrate animal, which is exceedingly doubtful. But the difficulty is to get to cephalopods.
It is confessedly easy to pass from the cœlenterate type to the annulose—or, in the old style, from Radiata to Articulata.
Nothing is more manifest in Nature than that she never loses the effect of a habit—never gives up a plan—never resigns the use of means and tools once adopted. When we have arrived at the involute cell as the type of the endothentic series—the animal kingdom—nothing is more obvious than the compounding of the simple form to constitute the rest of the kingdom. Nor is it necessary to suppose that the compounding takes place by any other means than that already observed in the case of exothentic creatures. If vegetal cells multiply and compound by a gemmating and fissiparous process, so also does the endothentic and celœnterate type.
Mr. Spencer has shown, triumphantly, how this may take place even mechanically. A simple endothen becomes two by ordinary growth, until division is forced at some median point, exactly as in the case of a cell. When the separation is complete we say this is an increase by gemmation. When there is differentiation without separation, we call it compounding—or, as Mr. Spencer has it, an aggregation. The same two great laws of aggregation and segregation, which rule in all things else, present themselves here also in explanation of the phenomena of life. As presented in annulose and annuloid creatures, the compounding is a segmentation. For an annulose or articulate segment is nothing else but one of the simple elements of a compound structure, of which the distinction of the parts is less pronounced. In sponges, in corals, in compound ascidians, the segregation is far advanced—the compounding is very evident. In these it is not denied any more than in the analogous compounding displayed in mosses, ferns, and trees. For, according to the laws of vegetal life already reviewed, every leaf, every node, is a distinct creature, and the bud of the node its progeny. But it is not at first sight so obvious that the real law of all creatures constituted of rings and joints is the law of compound association. It is difficult, without some reflection, to admit that every segment in these is a modification of the original unicellular creature—the mono-segmentarian from which the aggregation sprang. Yet the most casual anatomist cannot fail to perceive that each section in annulosa is but a repetition of the same structure with all its organs and appendages. Even where there are modification and differentiation of function, the structure is always and evidently identical—perfectly homologous.
Fully comprehending the nature of segments, we may proceed to the further efforts of Nature to obtain higher combinations and greater concentrations of energy. For this seems to be the end and object aimed at, if we may be allowed in our own minds to clothe Nature with conscious impersonation, i. e., personality. From the organization
