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the Hydractinea seems superior, inasmuch as it is itself a sexual form. The zone of spherical protuberances in the middle of the body are the ovaries or egg capsules corresponding to the sperm capsules of the male individual. Heterogenesis does not take place in our Hy

dractinia, but, as in the development of the ovum of the Cladonema into the Stauridium, there is a transformation of a ciliated lava into a sessile polype. But it is obvious that the part which in the Hydractinia is played by the male and female sexual organs is performed in the generative cycle of the Cladonema by the sexual animals. By following the transition from the dependent organ into the independent animal, we find the solution and explanation of the process termed heterogenesis. Between the genera reproduced like the Hydractinia, and those reproduced like the Cladonema, there are many others, of which the propagation shows the gradual transition of the rudimentary sexual organs into the sexual animal. We may so arrange the genera of the "Medusa polypes" as to exhibit how the parts which in the Hydractinia are mere capsules, generating and enclosing the ova, become more and more perfect. They acquire a special branch of the alimentary canal and blood-vessels, and are provided with the marginal papillæ characteristic of the Medusæ, and constituting their peculiar sensory organs. In short, what in one member of the systematic series may be termed an organ, is, in the next, the Medusa separating itself and

FIG. 15.

becoming a new generation; the sexual organ has become the sexual animal.

Now as the individual development of the Cladonema, and other Meduse similarly propagated, corresponds with the systematic series of the Medusa polypes, the only reasonable and credible explanation of the ontogenesis of those Medusæ in which heterogenesis occurs, is that, in them, the historical development of the genus has become fixed. Neither the egg nor the hen were created. Before the delicately tinted Medusæ populated the primæval ocean in lonely splendour, the Medusa polypes on the constantly changing shores were the sole representatives of the still infant class. Why single genera, like the Hydractinia, re- . mained strictly conservative while others in various degrees paid homage to progress, whether and how the struggle for existence and survival of the fittest were here concerned, it is certainly impossible to prove in the individual species. But the general impression is decisive, and also the circumstance that the theory is consistent with the facts.

The evolutionary history of the intestinal worms leads to the same reflections and results. These animals, widely differing in their structure, were either created in or with their hosts, or else they have become habituated to them in a natural and direct manner. We may surely disregard the third alternative, that they were led by an innate "obscure impulse." According to our doctrine, the worms now passing the whole or a portion of their lives as parasites on or in other organisms, are descended from free and independent animals, and the periods occurring in their development, during which parasitic life

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is exchanged for independent phases, signifies a reversion taking place systematically in all individuals to the once permanent condition of their progenitors. Of the Trematoda or Flukes, and Cestoda or Tapeworms, belonging to the class of the Platelmintha Suctoria, the latter have diverged the most from their starting-point; their adaptation to life within other animals has rendered the alimentary canal superfluous, and their generations and transformations hence point less to their progenitors than is the case with a number of other Trematoda, with which many anatomical characters prove them to be closely related. Both, moreover, share the characters of their class with the free-living Turbellaria. From such as these, that is to say, from forms approximate to the present Turbellaria, the Trematoda and Cestoda must be descended, and with this agrees the free roving phase which the larva of the Fluke (Distomum) undergoes as the so-called Cercaria, and previously as a rotating spherical body.

Many of the ciliated Nematoids, or thread-worms, too, the division which includes the Ascarides among others, have in their infancy a stage of independent life, during which they cannot be distinguished from the infantine forms of their more numerous kindred, which never adopt a parasitic life, and chiefly inhabit the sea. The transition to parasitism, as recapitulated by ontogenesis, was nothing more than an extension to a new territory offering advantages of nutriment; and on this point it is highly instructive to compare the Nematodes with the systematic series of the leechlike Suctoria (Trematoda), so excellently described by Van Beneden. We here find all the transitions

from independent predatory genera to others occasionally parasitic, and again from these to others which on leaving the egg immediately attach themselves for life. Here, as elsewhere, parasitism seems an adaptation to new habitats, which is recorded in the biography of the individual with a reminiscence of the previous form.

The circumstances of the parasitic worms are repeated by the parasitic Crustacea, as, moreover, a probably primordial form of the crab family is preserved in the metamorphoses of several orders of this large and diversified, though coherent class. The larva, which, it may safely be assumed, approximates closely to the primordial form, was at one time taken for an independent genus and received the name of Nauplius. Hence a Nauplius phase is spoken of, which obtains especially among the lower Crustacea, the Copepoda, parasitical Crustacea and Cirripedes, and the remarkable Rhizopoda connected with them; but is not wanting in the highest order, the decapodous stalk-eyed crab. We shall later have to make acquaintance with the so-called curtailed development which among the crabs has been adopted by the decapods, and it was formerly supposed by all. Were this actually the case, we should still, by analogy, infer their connection with the other orders repeating the Nauplius phase in the course of their development; but it was a welcome

FIG. 16.

discovery of Fritz Müller's that a shrimp (Peneus) still begins its development as a Nauplius; whereas all the other members of the order, as far as they are known, leave the egg in the higher Zoea phase (p. 50). As of the hundreds of stalk-eyed crabs, scarcely a dozen have been hitherto examined as to their development, it will not be doubted that, with regard to the Nauplius phase, some resemble the Peneus of the Brazilian coast. But even were this case to prove unique in the order, it would suffice as a living witness of the connection

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FIG. 17. Axolotl.

between the presence of the decapods and the primordial crabs. There can be no other view of this subject. The Nauplius phase in the development of the Peneus is either a shining testimony in favour of the doctrine of Descent, or a senseless paradox.

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