52 THE ORIGIN OP SPECIES 



to species having been exposed for long periods of time 

 to nearly uniform conditions of life. 



It is not surprising that the difficulty in crossing any 

 two species, and the sterility of their hybrid offspring, 

 should in most cases correspond, even if due to distinct 

 causes: for both depend on the amount of difference 

 between the species which are crossed. Nor is it sur- 

 prising that the facility of effecting a first cross, and the 

 fertility of the hybrids thus produced, and the capacity 

 of being grafted together — though this latter capacity 

 evidently depends on widely different circumstances- 

 should all run, to a certain extent, parallel with the 

 systematic affinity of the forms subjected to experiment; 

 for systematic affinity includes resemblances of all kinds. 



First crosses between forms known to be varieties, or 

 sufficiently alike to be considered as varieties, and their 

 mongrel offspring, are very generally, but not, as is so 

 often stated, invariably fertile. Nor is this almost uni- 

 versal and perfect fertility surprising, when it is remem- 

 bered how liable we are to argue in a circle with respect 

 to varieties in a state of nature; and when we remember 

 that the greater number of varieties have been produced 

 under domestication by the selection of mere external 

 differences, and that they have not been long exposed to 

 uniform conditions of life. It should also be especially 

 kept in mind that long-continued domestication tends to 

 eliminate sterility, and is therefore little likely to induce 

 this same quality. Independently of the question of 

 fertility, in all other respects there is the closest general 

 resemblance between hybrids and mongrels — in their 

 variability, in their power of absorbing each other by 

 repeated crosses, and in their inheritance of characters 



