Chap. VIII. UF FIRST CROSSES AND OF HYBRIDS. 249 



rcctly of hybrids, is simply incidental on unknow n ditFcrcnccs 

 in tli(^ reproductive sx'slems of the parent-species. 



We may now try and look a little closer at tlic probable 

 nature of these dilVerenies, Avhicli induce sterility in first crosses 

 and in hybrids. I*iire species and hybrids difl'er, as already re- 

 marked, in the state of their reproductive or<z;ans ; but from 

 what Avill presently follow on reciprocally dimorphic and tri- 

 inorphic plants, it would ajipear as if some unknown bond or 

 law existed, which causes the young from a union not fully 

 fertile to be themselves more or less infertile. 



In the case of first crosses between pure species, the greater 

 or less difiiculty in eflecting a union antl in obtaining offspring 

 aj^parently depends on several distinct causes. There must 

 sometimes be a ])hysical impossibilit}^ in the male element reach- 

 ing the ovule, as would be the case with a plant having a pistil 

 too long for tlie pollen-tubes to reach the ovarium. It has also 

 been observed that when 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 inca- 

 pable 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 caimot be grafted on others. Lastly, an embryo 

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

 latter alternative has not been sufHciently attended to ; but I 

 believe, fj-om observations communicated to me by Mr. Hewitt, 

 who has had great exjierience in hybridizing pheasants and 

 fowls, that the early death of the emliryo 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 b(itween three species of Gallus and their hy- 

 brids; the majority of these eggs had been fertilized; and in 

 the majority of the fertilized eggs, the embryos either had been 

 partially develojK'd and had then aborted, or had become nearly 

 mature, but the young chickens had been imable 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, " with- 

 out any obvious cause, apparently from mere inability to live;" 

 so that from the oOO eggs only twelve chickens were reared. 

 The early death of hybrid embryos jiroliably occurs in like 

 manner Avith plants; at least it is known that hybrids raised 

 from very distinct species arc sometimes weak and dwarfed, 



