Chap. VIII. OF FIRST CROSSES AND OF HYBRIDS. 241 



to be a sign of incipient fertilization. From this extreme degree 

 of sterility wc have self-fertilized hybrids jiroduting a greater 

 and greater number of seeds up to perfect fertility, 



Hyljrids from two species which are very difiicult to cross, 

 and which rarely produce any offspring, are generally very 

 sterile ; but the parallelism between the dilliculty of making a 

 first cross, and the sterility of the hybrids thus jiroduced — two 

 classes of facts which are generall}- confounded together — is by 

 no means strict. There are many cases, in Avhich two pure 

 species, :is in the genus Yerbascum, can be united with un- 

 usual facility, and produce numerous hybrid-offspring, yet these 

 hybrids are remarkabh' sterile. On the other hand, there are 

 sp(M-ies which can be crossed very rarelv, or with extreme diflli- 

 culty, but the hybrids, ■when at last produced, are very fertile. 

 Even within the limits of the same genus, for instance in Dian- 

 thus, these two opj^osite cases occur. 



• The fertility, both of first crosses and of hybrids, is more 

 easily affected by unfavorable conditions, than is the fertility 

 of pure species. But the degree of fertility is likewise innately 

 variable ; for it is not always the same when the same two 

 species arc crossed under the same circumstances, but depends 

 in part upon the constitution of the individuals which happen 

 to have been chosen for the experiment. So it is with hybrids, 

 for their degree of fertility is often found to differ greatly in 

 the several individuals raised from seed out of the same ca})sule 

 and exposed to the same conditions. 



By the term systematic affinity is meant, the resemblance 

 between species in structure and in constitution, more espe- 

 cially in the structure of parts which are of high physiological 

 importance and which differ little in the allied sjiecies. Now 

 the fertility of fust crosses between species, and of the hybrids 

 protluced from them, is largely governed by their systematic 

 affmity. This is clearly shown by hybrids never having lieen 

 raised l)etween species ranked by systematists in distinct fami- 

 lies ; and, on the other hand, by very closely-allied species gener- 

 ally uniting Mith facility. But the correspondence between 

 systematic affmit}- and the facility of crossing is by no means 

 strict, A nudtitude of cases could be given of very closely- 

 allied species which Avill not imite, or only with extreme diffi- 

 culty ; and, on the other hand, of very distinct species which 

 unite with the utmost facility. In the same family there may 

 be a genus, as Dianthus, in which very many species can most 

 readilv be crossed ; and another genus, as ISilcne, in which the 

 II 



