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characteristics of form and internal structure; and it is very easy to select from every type forms in which the distinctive marks, comprised in the systematic diagnosis, may be displayed in full perfection. But this is immediately succeeded by a further observation, that of gradations within the type. When we previously compared the polype and the bee, and were obliged to assign to each a very different rank, a portion of this difference of grade is certainly due to the difference of the family; but the forms united by family characteristics likewise diverge widely from each other, and the systematist speaks of lower and higher classes within every type, of lower and higher orders within every class.
Reason is compelled to this by the same considerations which forced themselves upon us in the comparison of the polype and the bee. Why does the mussel stand lower than the snail? Because it does not possess a head, because its nervous system is not so concentrated and so voluminous, because its sensory organs are more defective. In one, as in the other, the structural material is present in quantities sufficient for the completion of the type; but in the snail it is more developed, and the single circumstance of the integration of various parts to form the head confers a higher dignity upon the snail. It is needless to illustrate this gradation within the families by further examples; the most superficial comparison of a fish with a bird or a mammal, of one of the parasitic crustacea with a crayfish or an insect, shows, as the older zoology represented it, that in the actual forms the ground plan, or "ideal types," find very diversified expression.
A further result of this descriptive inquiry is the
tree-like grouping of the members of the same family. The reciprocal relations of the various families cannot be represented in a simple line; though in former days more importance was attributed to the general indications of the relative value of the types. On the other hand, descriptive zoology had long been compelled to devise tables of affinity for the systematic subdivisions, descending even to species according to the criterion of anatomical perfection; and these found expression only in diagrams of highly ramified trees. Branches appeared which terminated after a brief extension; others are greatly elongated with numerous side branches; in every branch characteristic phenomena and series are made manifest.
Let us attempt it with the Vertebrata, for example. Even with the fishes we fall into great perplexity; which to place at the end as being the highest. But take which we will, the sharks or our teleostei, the amphibians cannot be annexed in a direct line, nor does the elongated branch line of the latter merge, as might be imagined, into the reptiles. The birds, on their side, offer a sharp contrast to the mammals, and this separation and divergence extend to all the subdivisions. We must figuratively represent family branches, clusters of genera, and tufts of species, which latter ramify into sub-species and varieties. With this representation of the tree-like distribution of the system, we shall gladly revert to the comparison of the members of different types, with reference to their functional value. The bee in itself is manifestly a far more complex organism than the lowest fish-like animal, the lancelet; and in these two we compare a low form of a high type, and a high form
of a low type. By varying and combining comparisons of this sort, and taking account of the points of connection between the various types, to which we shall immediately refer, the figure of the systematic trees completes itself into one vast tree, of which the main branches are represented by the types.
Had the systematizers of the old school been familiar with the construction of plants and animals, they would have first established the diagnoses and distinctive characters, and then called to life the types and their species; for their chief torment has been, that the diagnoses are liable to so many exceptions, and that the characters of the fundamental forms are without any absolute value. Roughly and generally speaking, polypes are radiate in form, but not a few are bilateral, or symmetric on two sides. Most snails possess well-marked mantle-folds, but we can scarcely speak of the testa of many thoroughly worm-like slugs.
Head and skull seem an inalienable mark of the vertebrata, yet the lancelet has no such head, but merely an anterior end. Nevertheless, it may be objected, it has a vertebral column; yet this, the special badge of nobility of the vertebrate animals, like the auditory apparatus, and the notochord, is, even if only transiently, a possession of the Ascidians, a class of animals which in their mature condition do not bear the remotest resemblance to the Vertebrata. When we become aware of these deviations from so-called laws of form and structure, seemingly well established, we are prepared for a manifest failure of the system, in regard to connecting forms, and forms of uncertain position in the system.
If the result of the systematic sifting and arrangement within the individual types can be comprised in diagrams of trees, forms intermediate to the members of the types, classes, orders, &c., follow as a matter of course. For if the figure be correct, every ramification of the branches must include species diverging very slightly from the species standing in the lowest portions of the bough from which it branches off. And thus all systematizing, in fact, amounted to the insertion of the right intermediate forms between each two forms deviating from each other in a higher degree; nay, in some cases, intermediate forms were sought where none exist. The older zoology always regarded the duck-mole (Ornithorhynchus) as the mammal most nearly allied to the birds, though the cause of the bird-like appearance of the lowest mammal known, is by no means to be sought in a direct relationship, but in a remote cousinhood.
But we must draw attention, not to these connecting forms, which natural history assumes as perfectly selfevident, but to those which are, as it were, inconvenient to systematic description, and threaten to render illusory the groundwork so laboriously gained. There are some fish-like animals, the Dipnoi, (Lepidosirens and their congeners) with the characters of Amphibians. The Infusoria possess many characteristics of the so-called primordial animals, but in other ways they differ from them, and point to the lowest Turbellaria. A minute animal inhabiting our seas in countless multitudes, i.e. the Sagitta, is neither a true annelid nor a legitimate mollusc. The class of the Radiata fits neither into the system of the actual Annulosa, nor into that of the true Articulata, yet provision must be made for it in the
system; and any one who clings to the types as ideal and inalterable fundamental forms, falls into sad perplexity how to dispose of his Radiata.
Example after example might be thus accumulated to show that the rigid partitions of the system are scarcely raised before they are again broken down in every direction; and this in direct ratio with the increase of special science. As before said, descriptive natural history necessarily gained this experience. It then spoke of exceptions and deviations, without being able to adduce any reason why the classes and types should be able to break through their limits, and indeed most frequently without feeling any need of accounting for the failure of the rigid system.