Chap. IX. of 'first Crosses and of Hybrids. 249 



physical impossibility in the male element reaching the ovule, as 

 would be the case with a plant having a pistil too long for the pollen- 

 tubes to reach the ovarium. It has also been observed that when 

 the pollen of one species is placed on the stigma of a distantly allied 

 species, though the pollen-tubes protrude, they do not penetrate the 

 stigmatic surface. Again, the male element may reach the female 

 element but be incapable of causing an embryo to be developed, as 

 seems to have been the case with some of Thuret's experiments on 

 Fuci. No explanation can be given of these facts, any more than 

 why certain trees cannot be grafted on others. Lastly, an embryo 

 may be developed, and then perish at an early period. This latter 

 alternative has not been sufficiently attended to; but I believe, 

 from observations communicated to me by Mr. Hewitt, who has- 

 had great experience in hybridising pheasants and fowls, that the 

 early death of the embryo is a very frequent cause of sterility in 

 first crosses. Mr. Salter has recently given the results of an 

 examination of about 500 eggs produced from various crosses. 

 between three species of G alius and their hybrids ; the majority 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 mature, but the young chickens had 

 been unable to break through the shell. Of the chickens which 

 were born, more than four-fifths died within the first few days, or 

 at latest weeks, " without any obvious cause, apparently from mere 

 inability to live ; " so that from the 500 eggs only twelve chickens 

 were reared. With plants, hybridised embryos 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 partakes of only half of 

 the nature and constitution of its mother ; it may therefore before 

 birth, as long as it is nourished within its mother's womb, or within 



