On the slow and successive appearance of new species-On their different rates of change-Species once lost do not reappear— Groups of species follow the same general rules in their ap- pearance and disappearance as do single species-On extinction —On simultaneous changes in the forms of life throughout the world-On the affinities of extinct species to each other and to living species-On the state of development of ancient forms— Present distribution cannot be accounted for by differences in physical conditions-Importance of barriers-Affinity of the productions of the same continent-Centres of creation-Means of dispersal by changes of climate and of the level of the land, Distribution of fresh-water productions-On the inhabitants of oceanic islands—Absence of Batrachians and of terrestrial Mam- mals-On the relation of the inhabitants of islands to those of the nearest mainland-On colonisation from the nearest source with subsequent modification—Summary of the last and present CLASSIFICATION, groups subordinate to groups-Natural system- Rules and difficulties in classification, explained on the theory of descent with modification-Classification of varieties-De- scent always used in classification—Analogical or adaptive char- acters -Affinities, general, complex, and radiating-Extinction separates and defines groups-MORPHOLOGY, between members of the same class, between parts of the same individual— EMBRYOLOGY, laws of, explained by variations not super- vening at an early age, and being inherited at a correspond- Recapitulation of the objections to the theory of Natural Selection -Recapitulation of the general and special circumstances in its favour-Causes of the general belief in the immutability of species-How far the theory of Natural Selection may be ex- tended-Effects of its adoption on the study of Natural History ORIGIN OF SPECIES. CHAPTER IX. HYBRIDISM. Distinction between the sterility of first crosses and of hybrids— Sterility various in degree, not universal, affected by close interbreeding, removed by domestication-Laws governing the sterility of hybrids-Sterility not a special endowment, but incidental on other differences, not accumulated by natural selection-Causes of the sterility of first crosses and of hybrids —Parallelism between the effects of changed conditions of life and of crossing-Dimorphism and trimorphism-Fertility of varieties when crossed and of their mongrel offspring not universal-Hybrids and mongrels compared independently of their fertility-Summary. THE view commonly entertained by naturalists is that species, when intercrossed, have been specially endowed with sterility, in order to prevent their confusion. This view certainly seems at first highly probable, for species living together could hardly have been kept distinct had they been capable of freely crossing. The subject is in many ways important for us, more especially as the sterility of species when first crossed, and that of their hybrid offspring, cannot have been acquired, as I shall show, by the preservation of successive profitable 1 degrees of sterility. It is an incidental result of differences in the reproductive systems of the parentspecies. In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been confounded; namely, the sterility of species when first crossed, and the sterility of the hybrids produced from them. Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offspring. Hybrids, on the other hand, have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals; though the formative organs themselves are perfect in structure, as far as the microscope reveals. In the first case the two sexual elements which go to form the embryo are perfect; in the second case they are either not at all developed, or are imperfectly developed. This distinction is important, when the cause of the sterility, which is common to the two cases, has to be considered. The distinction probably has been slurred over, owing to the sterility in both cases being looked on as a special endowment, beyond the province of our reasoning powers. The fertility of varieties, that is of the forms known or believed to be descended from common parents, when crossed, and likewise the fertility of their mongrel offspring, is, with reference to my theory, of equal importance with the sterility of species; for it seems to make a broad and clear distinction between varieties and species. Degrees of Sterility.-First, for the sterility of spe |