Seite - 315 - in The Origin of Species

CAUSES OF THE STERILITY 315 crosses. Mr. Salter has recently given the results of an ex- amination of about 500 eggs produced from various crosses between three species of Gallus and their hybrids; the ma- jority of these eggs had been fertilised; and in the majority of the fertilised eggs, the embryos had either been partially developed and had then perished, or had become nearly ma- ture, but the young chickens had been unable to break through the shells. Of the chickens which were born, more than four- fifths died within the first few days, or at latest weeks, "with- out any obvious cause, apparently from mere inability to live;" so that from the 500 eggs only twelve chickens were reared. With plants, hybridised embpyos probably often perish in a like manner; at least it is known that hybrids raised from very distinct species are sometimes weak and dwarfed, and perish at an early age; of which fact Max Wichura has recently given some striking cases with hybrid willows. It may be here worth noticing that in some cases of parthenogenesis, the embryos within the eggs of silk moths which had not been fertilised, pass through their early stages of development and then perish like the embryos produced by a cross between distinct species. Until becoming acquainted with these facts, I was unwilling to believe in the frequent early death of hybrid embryos; for hybrids, when once born, are generally healthy and long-lived, as we see in the case of the common mule. Hybrids, however, are differently cir- cumstanced before and after birth ; when born and living in a country where their two parents live, they are generally placed under suitable conditions of life. But a hybrid par- takes of only half of the nature and constitution of its mother ; it may therefore before birth, as long as it is nour- ished within its mother's womb, or within the egg or seed produced by the mother, be exposed to conditions in some de- gree unsuitable,and consequently be liable to perish at an early period; more especially as all very young beings are eminently sensitive to injurious or unnatural conditions of life. But af- ter all, the cause more probably lies in some imperfection in the original act of impregnation, causing the embryo to be imperfectly developed, rather than in the conditions to which it is subsequently exposed. In regard to the sterility of hybrids, in which the sexual