which they are subdivided similarly become differentiated
and coalesce.It is in virtue of such processes that the flowers of plants, and the heads and limbs of the Arthropoda and of the Vertebrata, among animals, attain their extraordinary diversity and complication of structure. A flower-bud is a segmented body or axis, with a certain number of whorls of appendages ; and the perfect flower is the result of the gradual differentiation and confluence of these primitively similar segments and their appendages. The head of an insect or of a crustacean is, in like manner, composed of a number of segments, each with its pair of appendages, which by differentiation and confluence are converted into the feelers and variously modified oral appendages of the adult.
In some complex organisms, the process of differentiation, by which they pass from the condition of aggregated embryo cells to the adult, can be traced back to the laws of growth of the two or more cells into which the embryo cell is divided, each of these cells giving rise to a particular por tion of the adult organism. Thus the fertilized embryo cell in the archegonium of a fern divides into four cells, one of which gives rise to the rhizome of the young fern, another to its first rootlet, while the other two are con verted into a placenta-like mass which remains embedded in the prothallus.
The structure of the stem of Chara depends upon the different properties of the cells, which are successively derived by transverse division from the apical cell. An inter-nodal cell, which elongates greatly, and does not divide, is succeeded by a nodal cell, which elongates but little, and becomes greatly subdivided ; this by another inter-nodal cell, and so on in regular alternation. In the game way the structure of the stem, in all the higher plants, depends upon the laws which govern the manner of division and of metamorphosis of the apical cells, and of their continuation in the cambium layer.
In all animals which consist of cell-aggregates, the cells of which the embryo is at first composed arrange them selves by the splitting, or by a process of invagination, of the blastoderm into two layers, the epiblast and the hypoblast, between which a third intermediate layer, the mesoblast, appears, and each layer gives rise to a definite group of organs in the adult. Thus, in the Vertebrata, the epiblast gives rise to the cerebro-spinal axis, and to the epidermis and its derivatives ; the hypoblast, to the epithelium of the alimentary canal and its derivatives ; and the mesoblast, to all the intermediate structures. The tendency of recent inquiry is to prove that the several layers of the germ evolve analogous organs in invertebrate animals, and to indi cate the possibility of tracing the several germ layers back to the blastomeres of the yelk, from the subdivision of which they proceed.
It is conceivable that all the forms of life should have presented about the same differentiation of structure, and should have differed from one another by superficial characters, each form passing by insensible gradations into those most like it. In this case Taxonomy, or the classification of morphological facts, would have had to confine itself to the formation of a serial arrangement representing the serial gradation of these forms in nature.
It is conceivable, again, that living beings should have differed as widely in structure as they actually do, but that the interval between any two extreme forms should have been filled up by an unbroken series of gradations ; in which case, again, classification could only effect the formation of series the strict definition of groups would be as impossible as in the former case.
As a matter of fact, living beings differ enormously, not only in differentiation of structure, but in the modes in which that differentiation is brought about ; and the intervals between extreme forms are not filled up in the existing world by complete series of gradations. Hence it arises that living beings are, to a great extent, susceptible of classification into groups, the members of each group resembling one another, and differing from all the rest, by certain definite peculiarities.
No two living beings are exactly alike, but it is a matter of observation that, among the endless diversities of living things, some constantly resemble one another so closely that it is impossible to draw any line of demarcation between them, while they differ only in such characters as are associated with sex. Such as thus closely resemble one another constitute a morphological species ; while different morphological species are defined by constant characters which are not merely sexual.
The comparison of these lowest groups, or morphological species, with one another, shows that more or fewer of them possess some character or characters in common some feature in which they resemble one another and differ from all other species and the group or higher order thus formed is a genus. The generic groups thus constituted are susceptible of being arranged in a similar manner into groups of successively higher order, which are known as families, orders, classes, and the like.
The method pursued in the classification of living forms is, in fact, exactly the same as that followed by the maker of an index in working out the heads indexed. In an alphabetical arrangement, the classification may be truly termed a morphological one, the object being to put into close relation all those leading words which resemble one another in the arrangement of their letters, that is, in their form, and to keep apart those which differ in structure. Headings which begin with the same word, but differ otherwise, might be compared to genera with their species ; the groups of words with the same first two syllables to families ; those with identical first syllables to orders ; and those with the same initial letter to classes. But there is this difference between the index and the Taxonomic arrangement of living forms, that in the former there is none but an arbitrary relation between the various classes, while in the latter the classes are similarly capable of co-ordination into larger and larger groups, until all are comprehended under the common definition of living beings.
The differences between "artificial " and "natural " classi fications are differences in degree, and not in kind. In each case the classification depends upon likeness ; but in an artificial classification some prominent and easily observed feature is taken as the mark of resemblance or dissemblance ; while, in a natural classification, the things classified arc arranged according to the totality of their morphological resemblances, and the features which are taken as the marks of groups are those which have been ascertained by ob servation to be the indications of many likenesses or un- likenesses. And thus a natural classification is a great deal more than a mere index. It is a statement of the marks of similarity of organization ; of the kinds of struc ture which, as a matter of experience, are found universally associated together; and, as such, it furnishes the whole foundation for those indications by which conclusions as to the nature of the whole of an animal are drawn from o knowledge of some part of it.
bone or of a shell to the nature of the animal to which that bone or shell belonged, he is guided by the empirical morphological laws established by wide observation, that such a kind of bone or sJiell is associated with such and
such structural features in the rest of the body, and no