whereas the habit of the series of individuals is incredibly pliable. This pliability of the whole body is not lacking in the silicious sponges; in the genus Tedania, for instance, established by Gray from some of my earlier Reniera, we see how their stubbornly coherent needlelike forms recur from Trieste to Florida and Iceland, under the most heterogeneous disguises. In some varieties, however, one of these spicula already manifests a tendency to deviations.

This very point, the possibility of tracing in detail the metamorphoses of organs, which, on the assumption of their stability, appeared to provide the system with the most substantial basis for the erection of genera and species, renders the investigation peculiarly attractive. Even among the Algierian sponges, I have adduced striking examples, and they accumulate in proportion as the horizon is extended. We arrive gradually at the conviction that no reasonable dependence can be placed on any "characteristic;" that with a certain constancy in microscopic constituents, the outward bodily form, with its coarser distinctive marks, varies far beyond the limits of the so-called species and genera; and that, with like external habits, the internal particles, which we looked upon as specific, are transformed into others, as it were, under our hands. "Any one"-thus concludes this section of my work on the Fauna of the Atlantic Sponges,-" who, with regard to sponges, makes his chief business the manufacture of species and genera, is reduced ad absurdum, as Haeckel has shown with exquisite irony in his Prodrome to the Monograph on the Calcareous Sponges."

In my specific rescarches I confined myself essentially

1

to the silicious sponges, and by thousands of microscopic observations, by measurements, by drawings, by facts and inferences, had produced evidence, which acute opponents of the immutability of species had not brought forward before me, that in these sponges, species and genera, and consequently fixed systematic unities in general, had no existence. The other division of the same class, the calcareous sponges, had been treated with unrivalled mastery by Haeckel in his Monograph.23

He was able not only to confirm my statements, but, owing to the smaller compass and the greater facility of observing the group selected for study, to advance with more sequence and continuity from the observation of details to the whole, to portray its morphology, physiology, and evolutionary history with the utmost completeness. He then challenged the obstructive party with the assertion that, according to subjective opinion, either one or 591 species of calcareous sponges might be accepted, but “that no absolute species exists, and that species and varieties cannot be sharply separated." Whoever after these demonstrations cleaves to the phantom of species, without either proving that the facts have been falsely observed, or that they may be interpreted otherwise than in favour of the stability of species,-whoever, as Agassiz has recently done, ignoring any such researches, publicly asseverates that in no single case has the mutability of any species been exhibited, scarcely preserves the right to participate in the great controversy by which Natural Science is now perturbed.

There is, however, as we have already mentioned, a second direction in which the mobility of "species" must

be demonstrated, not the direction of breadth, but of height and depth. This mutability of the Spongiada affords the extremely important evidence that, so to speak, an entire class has, even now, not attained a state of comparative repose. But to confirm the mutability of species, evidence of mutability in lapse of time is justly demanded; the transition of the forms succeeding one another historically in the strata of the earth.

A highly instructive example of the transmutation of species occurring in the lapse of time, and one which may at all events be banished from the limits of varieties, is offered by the Tellina (Planorbis multiformis) occurring in the fresh-water chalk of Steinheim, in Würtemberg. The deposit, derived from the Tertiary period, contains the residue of a small lake, and may be divided into about 40 petrographically distinguishable layers. "In the whole series of strata," says Hilgendorf," "the varieties of Planorbis multiformis are distributed in such a manner that individual layers are characterized as successive strata, by the exclusive occurrence or by the predominance of single or several varieties which, within the layer, remain constant or slightly variable, but towards the limits of the next layer, lead by transitions to the succeeding forms. The intermediate layers furnish evidence that the other forms originated by gradual metamorphosis from the earlier ones; they moreover render it possible to range form to form, and to trace the evolution backwards; hence it becomes manifest that what above seemed distinctly divided, meets below. Thus arises a pedigree richly endowed with main and side branches." The forms diverge so greatly, and are so constant in the main zones, which tell of perio ls of

repose, that, in accordance with the old conchological practice, they would be unreservedly claimed as species, if the connecting links were not too conspicuous and the territory too circumscribed, and if the geological period, which must, however, be reckoned at least by thousands of years, were not considered too insignificant.

But what the case of Steinheim exhibits in miniature was taking place on a large scale during the great geological periods, and the zeal of some Palæontologists, such as Waagen, Zittel, Neumayr, Würtenberger," has had the effect of proving, at least with respect to the important division of the Ammonites, the utter impossibility of separating them into "species." The study of Ammonites has shown that from fossil remains important inferences may be made as to the whole organization, and that, in combination with the observable modifications of the shell, simultaneous and profound metamorphoses of certain determining soft parts must have taken place. If it is now proved, as it has been by these investigators, that the so-called "species" which characterize the great Jurassic and cretaceous formations, are connected in the same manner as the varieties of the Steinheim snail, as mere morphological series of variable constancy and duration, they who will not allow even this evidence to rouse them from their innate drowsiness, are like the ostrich which prefers not to see the danger. Neumayr is such a cool and cautious observer, that he allows nothing to pass current but that which is absolutely certain. It is true he holds it to be "extraordinarily probable" that in all forms these gradual transitions have taken place, yet in one case only does he demand unqualified assent, namely,

that he has proved" that Perisphinctes aurigerus (Opp.) of the Bathonians, and Perisphinctes curvirostrus of the zone of the Cosmoceras Jason (Rein), are connected in such a manner by intermediate occurrences that it is impossible to draw a limit.

L. Würtenberger applied his researches to thousands of samples from the groups of the Planulate Ammonites with ribbed shells, and of the Armate Ammonites with prickly shells. In summing up his results he says, among other things: "How among the Ammonites of the Planulate and Armate groups, the species are to be branched off from one another, I should be reluctant and unable to give any instructions, for to me this question appears utterly hopeless. For in groups of fossil organisms, in which, as in the present case, so many connecting links between the most extreme forms are actually before us, that the transition is regularly carried on, the species is far less susceptible of apprehension than in the organic forms of the present world, which at least denote the existing limits of the great pedigree of the organic world. With respect to these fossil forms, it is fundamentally indifferent whether a very short, or a somewhat longer portion of any branch be honoured by a special name, and looked upon as a species. The prickly Ammonites, classified under the name of Armata, are so intrinsically connected, that it becomes an impossibility to separate the accepted species sharply from one another. The same observation applies also to the group of which the manifold forms are distinguished by their ribbed shells, and termed Planulata." It has further transpired that the Armata, or Custata, originated from the Planulata.