« ПредыдущаяПродолжить »
We shall return later to Würtenberger's preliminary communications. It was our object here to inform our readers how and where modern natural inquiry sets aside the phantom of species, and to enable them to judge for themselves what series of observations are opposed to the asseverations that in no single case has evidence been given of the transition of one species into another. For the old school falls into the dilemma of proclaiming whole orders and classes to be "species," and the species, formerly so beautifully defined, to be varieties.
The untenableness of the physiological part of the definition of species has been conclusively shown first by Darwin and afterwards by Haeckel. It is known that even in a state of freedom good species not infrequently breed together, and that domesticated species, such as the horse and the ass, have been crossed for thousands of years. But hybrids, the produce of this intercourse, were supposed to be only exceptionally fertile, and at any rate not to produce fertile progeny for more than a few generations. On the other hand, it was considered certain that the produce of crosses among varieties are fertile in unbroken succession. The dogma of the sterility of hybrids was formed without any experimental or general observation, and by ill-luck was apparently confirmed by the most ancient and best known hybridizations of the mule and the hinny. To this familiar example, in which the fertility of hybrids proves abortive, we will oppose only one case of propagation successfully accomplished in recent times through many generations; that, namely, of hares and rabbits, two "good species' never yet regarded as mere varieties.
The numerous and varied forms of the domestic dog
were pronounced ex cathedra to be varieties of the same species, as their crosses are productive. But after reading Darwin's careful comparison of the reports as to the relations of certain species of wolves with the dogs of savage nations, and of the European wolf with the Hungarian dog, we must agree with Darwin in thinking it as extremely probable that in various parts of the world, and at various periods, wild species of the genus Canis were domesticated, of which the crosses produce fertile progeny to an extent almost unlimited.
It is the same with the domestic cat. With the forms of the European domestic cat, the case is such that it is scarcely possible to doubt its origin partly from a Nubian species, and partly from the European wildcat. The inferences thus moved in a circle; forms belong to the same species, because they may be fruitfully crossed; and because they may be fruitfully crossed, they belong to the same species; and, on the other hand, because such and such forms, when crossed, produce no fertile progeny, they constitute different species; and because they are different species, they generate no fertile offspring. The cases of persistent fertility in hybrids are certainly not frequent, but they are nevertheless so well certified that the contrary statement is in plain contradiction to the facts. But conversely, the proposition that mongrels, the products of crosses among varieties, are fertile, thus generally stated, is likewise untenable. The variety which has been evolved in Paraguay from our domestic cat, pairs no longer with its ancestral stock, nor does the tame European guinea-pig with the wild ancestral stock of Brazil.
But even if, in general, crosses between varieties are
more easily effected, and more often produce fertile offspring than the unquestionably rarer crosses of species the frequent failure of crosses between species completely accords with the modification of species in the lapse of time, as shown above. Provisionally, let us hold nothing to be established but that, as to fertility and the capability of persistent reproduction, the conditions of mongrels and of hybrids are essentially similar and differ only in degree, and that on these properties, no closer definition or limitation can be founded.
If the older definitions of species go back to Paradise, and derive existent species lineally from ancestral progenitors, miraculously created from the first and never modified, the ingenuous statements of Linnæus show that all this was accepted as self-evident, and that no thought was given to the proof, which would indeed have been impossible to obtain. A letter from George Forster to Peter Camper, dated May 7th, 1787, proves however that, even in the last century, the voices of more farsighted naturalists were raised against this superficial treatment of the idea of species. Systems, he said, were founded on this idea, yet everything was uncertain as long as this expression was not irremovably fixed. But hitherto all definitions of this word were hypothetical, and in themselves anything but clear. If we are to accept as many species as were created, how is a created species to be distinguished from one produced by the intermixture of several others? To fall back upon the Creation is to lose oneself in the Infinite and the Impalpable. "This will never enable us to understand anything; and definitions which rest on an
inexplicable foundation, on a mystery, ought to be proscribed from science for evermore."
Without owning allegiance to any theory whatever, we are constrained to recognize the fact, that in various groups of organisms there even now exists such an instability of form, and such a degree of variability, that it is patent how constrained and artificial is their systematic separation. In many other groups, in most orders of the Mammalia, for example, this phase of mobility has been replaced by a certain quiescence, and the forms now presenting themselves for observation and comparison are so well defined from one another, that they fit into the system without difficulty as "good species." But if the "good species" are to be judged by the experiences made in regard to the "bad" ones, and if the preposterous hypothesis is not laid hold of, in contravention to all healthy human understanding, that "good species" originated in a miraculous manner inaccessible to our cognition, whereas the "bad species" are susceptible of analysis,-the other alternative alone is possible, that, as Haeckel says, if we knew them thoroughly, all species without exception would, in the sense of the species-makers, be “bad species." We are also acquainted with a sufficient number of bad species to be capable of inferring the general law with certainty. Nevertheless, all further corroboration and discovery of bad species is acceptable. Regarded formerly by the systematists only as incumbrances and as stones rejected by the builders, they have now become the corner-stones of science.
Is species therefore, we again inquire, to be entirely abandoned? Not so, for several reasons. Even assuming
that so-called good species, in the sense of the systematists, have no existence, human intellect, in the endeavour to obtain a general view, would be compelled to denominate the forms, unless all scientific treatment was to be rendered impracticable. But the retention of species is moreover scientifically justifiable and necessary, if only the determining impulses be taken into account, and the definition reduced to harmony with reality. Species is not constituted merely of analogous individuals, for even the sexes, in the course of development, and without transformation, diverge considerably from one another.
But if we remember the transmutation of shape taking place by stages in organisms subject to metamorphosis, and the regular sequence of forms alternating with one another in heterogenesis, we shall be obliged to speak, not of individuals, but of the cycles of reproduction which comprise the various phases and series of individuals. These remain persistent as long as they exist under the same external conditions. How far time in itself affects existence and decay is unknown. At any rate, time, as well as the external conditions of time, is a factor in the mutation of species. While we regard species as absolutely mutable, and only relatively stable, we will term it, with Haeckel, “the sum of all cycles of reproduction which, under similar conditions of existcnce, exhibit similar forms."