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closely compared with the mouth of the polype and medusa, and the great central cavity of the sponge with the stomach of the others, of the canal system with the canals and cavities of the Cœlenterata,—then, in combination with the host of other facts, implying and supporting the doctrine of Descent, the inference is inevitable that in the Gastrula we have a testimony of the consanguinity of the Spongiada and Colenterata. But this Gastrula reappears in the Holothuria ; hence in the Echinoderms, in the Sagitta, in the Ascidians, which will be more narrowly examined in the pedigree of the Vertebrata, and finally in the Lancelet; and we, therefore,, hold ourselves justified in regarding this coincidence of the earliest states of development in different families, as the remnant of the common root, which in other families, as in the Articulata, for example, has been lost in the curtailment of development. The significance of the "germinal membranes" in the Vertebrata was recognized even by Pander, and in the suggestive works of v. Baer; the extension and application of this observation to the whole animal kingdom, for which we are especially indebted to Kowalewsky, marks one of the greatest advances in the science of comparative development.
The reader unacquainted with the detailed researches of our science, has already been called upon to observe that there are opponents of the theory of selection, such as Owen, who nevertheless accept the doctrine of Descent as incontestable. Even rejecting natural selection, the parallelism of Ontogenesis with Phylogenesis may also be brought into the natural connection maintained by
us, on the assumption of an unnatural or supernatural guidance which converts this apparently natural unity into a miracle. Quite recently, A. Braun has pointed out the accordance of the botanical system, and therewith of paleontological succession, with the development of the individual plant, when he says:-"In the further elaboration of the natural system, the gradation of the vegetal kingdom, and, at the same time, the relation of the system to the history of development, becomes more and more spontaneously and incontrovertibly manifest. The Acotyledons are verified as Cryptogams, as they were already considered by the old botanists of preLinnæan times, and their relation to the Phænogams is thus more clearly pronounced. The Cryptogams are separated into two essentially different divisions, in which gradation is likewise distinctly pronounced (cellular and vascular Cryptogams, Thallophytes and Kormophytes); between the perfect Phænogams and the Cryptogams an intermediate grade has been shown, that of the Gymnosperms. But most important of all is the circumstance that the four chief grades ascertained in the vegetal kingdom accurately correspond with the grades of development occurring in the individuals of all the higher plants;—the germ, the vegetative stem, the blossom and the fruit." But why this parallelism is to be most important of all, if it is not to lead us to the knowledge of true causality, is beyond our comprehension. We can well imagine that the "inherent causes" and the "Principle of Perfection" may be welcomed as the refugium ignorantiæ, but not that they can really satisfy inquiry. For our own standpoint, the accordance of the results of botanical investigation
must be extremely important, but it is for the palpable reason that the theory thereby gains the support and corroboration of another great series of facts.
If the accordance of the evolution of families has once been followed up to the Gastrula, we shall not pause there, but must regard the similarity of the sperm corpuscules and germ cells from the Spongiada to the Vertebrata as a primordial common property, connecting the animal and vegetal world; and prior to the acquisition of which, only those modes of reproduction took place which have been maintained among Protista and in heterogenesis.
As the common basis of sexual reproduction in the various families argues a common origin, asexual reproduction, directly connected as we have seen it to be with sexual propagation, by means of unfecundated eggs and germs, leads us constantly further towards the beginning of life. But the cell furnished with a nucleus and sheath is inseparable from the protoplasmic corpuscule devoid of nucleus or sheath, on the growth and fission of which rests the reproduction of the lowest living beings.
Their origin from inorganic matter, as we have set forth above, is a postulate of sound human understanding. To this beginning we are led, not, as the opponents of the doctrine of Descent are wont to say, by a dogmatic after-philosophy, but by the unprejudiced consideration and computation of the facts of individual development.68
The Geographical Distribution of Animals in the light of the Doctrine of Derivation.
ALTHOUGH ever since the century of the great geographical discoveries, material has been accumulating for a geography of plants and animals, the foundations of scientific botanical geography (apart from George Forster's observations) were first contained in Hum- . boldt's celebrated "Ideas on the Physiognomy of Plants" (Ideen zu einer Physiognomik der Gewächse). It is the first description of vegetal forms, comprising the entire area of the earth, and the manner in which, singly or combined, they lend a characteristic impress to the landscape of their region of distribution, and again on their side harmonize with the other factors of the scene. The celebrated founder of Climatology, who circled the terrestrial globe with lines of equal temperature, of equal inclination and declination of the magnetic needle, and divided it into dry and rainy zones, knew better than any of his contemporaries that the animal and vegetal world depended on all these factors. Yet neither he nor his followers, before Darwin, rose higher than the description of Nature, which had already checked Buffon in his grand picture of Nature, "Les Epoques de la Nature.”
A natural result of the extraordinary extension of
the geographical horizon and the profundity of special research was the more careful ascertainment of the regions of distribution of animal and vegetal families, and of their more prominent species, in which, as we have already said, either no questions were asked as to the causes of distribution, or the matter was facilitated, as by Louis Agassiz, who did not, like Linnæus, derive each species from a pair, but supposed them to be created in suitable numbers of individuals in their own
regions of distribution. It cannot be expected that any solution was hereby given to the questions which now force themselves upon us, such as why, under like natural conditions, like species are not always to be found, and conversely? Why very similar species frequently appear under external conditions entirely dissimilar? What is to be thought of the mutual relations of the so-called vicarious forms? &c.
As Rütimeyer has recently observed, in his excellent treatise "On the Derivation of the Animal World of Switzerland" ("Ueber die Herkunft der schweizerischen Thierwelt" 6), Buffon had already remarked the repetition of the African in the American fauna; how, for example, the lama is a juvenescent and feeble copy of the camel; and how the puma of the New represents the lion of the Old World. Still, by the mere word "representative" or "vicarious form" nothing is gained, and a true apprehension of these facts is obtained singly and solely if we meet the inquiry with the assumption that camel and lama, puma and lion, are of common derivation, and that their diverse development was in the lapse of time favoured and determined by the separation of the habitats of their progenitors.