Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/438

422 422 H Y B H Y B separate the history of these two cities ; and no doubt the ancient writers themselves often do not clearly distinguish between them. The Hyblaean honey, which was produced on the hills beside them, is often celebrated by the Latin poets. There was a third city of the same name on the road from Syracuse to Agrigeutum. HYBRIDISM. The Latin word hybrida, or hibrida, a hybrid or mongrel, is commonly derived from the Greek v/3pis, an insult or outrage, with special reference to lust ; hence an outrage on nature, a mongrel. As a general rule animals or plants belonging to distinct species are not able, when crossed with each other, to pro duce offspring. There are, however, innumerable excep tions to this rule ; and hybridism is the word employed to denote these exceptions. It is an abstract term which signifies the more or less fertile crossing of distinct species. In scientific usage, the term &quot; hybrid &quot; is exclusively reserved to denote the result of a fertile cross between two distinct species, while the term &quot; mongrel &quot; is the one which is as exclusively reserved to denote the result of a fertile cross between two varieties of the same species. Until recently the interest attaching to hybridism was almost entirely of a practical nature, and arose from the fact, which is of considerable importance in horticulture, th it hybrids are often found to present characters somewhat different from those of either parent species. But of late years the subject has acquired a high degree of scientific interest in relation to the theory of descent. On this account it has been so carefully and thoroughly treated by Mr Darwin that a brief exposition of its main facts and principles must necessarily be little more than a condensation of his already closely packed material. Looking first to the general facts and principles of hybridism, apart from their bearing upon the theory of descent, the following may be regarded as the most im portant : 1. The laws governing the production of hybrids are identical, or nearly identical, in the animal and vegetable kingdoms. 2. The sterility which so generally attends the crossing of two specific forms is to be distinguished as of two kinds, which, although often confounded by naturalists, are in reality quite distinct. For the sterility may obtain between the two parent species when first crossed, or it may first assert itself in their hybrid progeny. In the latter case the hybrids, although possibly produced without any appearance of infertility on the part of their parent species, nevertheless prove more or less infertile among themselves, and also with members of either parent species. 3. The degree of both kinds of infertility varies in the case of different species, and in that of their hyl&amp;gt;rid progeny, from absolute sterility up to complete fertility. Thus, to take the case of plants, &quot; when pollen from a plant of one family is placed on the stigma of a plant of a distinct family, it exerts no more influence than so much inorganic dust. From this absolute zero of fertility, the pollen of different species, applied to the stigma of some one species of the same genus, yields a perfect gradation in the number of seeds produced, up to nearly complete, or even quite complete, fertility; so, in hybrids themselves, there are some which never have produced, and probably never would produce, even with the pollen of the pure parents, a single fertile seed ; but in some of these cases a first trace of fertility may be detected, by the pollen of one of the pure parent species causing the flower of the hybrid to wither earlier than it otherwise would have done ; and the early withering of the flower is well known to be a sign of incipient fertilization. From this extreme degree of sterility we have self- fertilized hybrids producing a greater and greater number of seeds up to perfect fertility.&quot; 4. Although there is, as a rule, a certain parallelism, there is no fixed relation between the degree of sterility manifested by the parent species when crossed and that which is manifested by their hybrid progeny. There are many cases in which two pure species can be crossed with unubual facility, while the resulting hybrids are remark ably sterile ; and, contrariwise, there are species which can only be crossed with extreme difficulty, though the hybrids, when produced, are very fertile. Even within the limits of the same genus, these two opposite cases may occur. 5. When two species are reciprocally crossed, i.e., male A with female B, and male B with female A, the degree of sterility often differs greatly in the two cases. The sterility of the resulting hybrids may differ likewise. 6. The degree of sterility of first crosses and of hybrids runs, to a certain extent, parallel with the systematic affinity of the forms which are united. &quot; For species belonging to distinct genera can rarely, and those belong ing to distinct families can never, be crossed. The parallelism, however, is far from complete ; for a multitude of closely allied species will not unite, or unite with extreme difficulty, whilst other species, widely different from each other, can be crossed with perfect facility. Nor does the difficulty depend on ordinary constitutional differ ences ; for annual and perennial plants, deciduous and evergreen trees, plants flowering at different seasons, in habiting different stations, and naturally living under the most opposite climates, can often be crossed with ease. The difficulty or facility apparently depends exclusively on the sexual constitution of the species which are crossed, or on their sexual elective affinity.&quot; Such being the principal facts of hybridism, we may next consider the relation which they bear to the theory of descent. It is obvious that the most important point of contact between the former and the latter consists in this that, although hybridism is occasionally possible as an exception to the general infertility of species inter se, it is only, as it were, a [/artial exception ; for, even when pro duced, the hybrid progeny almost invariably manifest some greater or less degree of sterility, and this not only when crossed among themselves, but even when crossed with either of their parent species. The main facts of hybridism thus at first sight seem to support the time-honoured doctrine that there are placed between all species the barriers of mutual sterility, for the purpose of preventing any admixture of specific qualities by heredity, and so for the purpose of maintaining the immutability of specific types. And the apparent support which this doctrine thus receives from the main facts of hybridism is still further strengthened when these facts are contrasted with those which are supplied by the breeding of our domestic &quot;varieties.&quot; For, in the latter case, and as an almost invariable rule, neither the organisms when crossed nor their resulting progeny show any indications of sterility, although the two parent varieties may differ from one another even more widely than do many natural species which are wholly infertile when crossed. This very general distinction between natural species and domestic varieties has appeared to many competent persons in the present generation so profound and significant that they deem it to be in itself sufficient to discredit, if not to negative, the whole theory of the transmutation of species. Now, when this distinction is thus posited as an objection to the theory of descent, we must first of all remember that this theory does not require the possibility of the com mingling of specific types ; it requires, indeed, that specific types should not be immutably fixed, but it does not require that the causes of their mutation should depend upon their mutual crossing. The whole difficulty, therefore, which