296 EVOLUTION [CHAP. IV 



Letter 213 as crossing, but this does not seem to me to weaken the 

 case as one of analogy. The incapacity of grafting is like- 

 wise an invariable attribute of plants sufficiently remote 

 from each other, and sometimes of plants pretty closely 

 allied. 



The difficulty of increasing the sterility through Natural 

 Selection of two already sterile species seems to me best 

 brought home by considering an actual case. The cowslip 

 and primrose are moderately sterile, yet occasionally pro- 

 duce hybrids. Now these hybrids, two or three or a dozen 

 in a whole parish, occupy ground which might have been 

 occupied by either pure species, and no doubt the latter 

 suffer to this small extent. But can you conceive that 

 any individual plants of the primrose and cowslip which, 

 happened to be mutually rather more sterile (i.e. which, 

 when crossed, yielded a few less seed) than usual, would 

 profit to such a degree as to increase in number to the 

 ultimate exclusion of the present primrose and cowslip? I 

 cannot. 



My son, I am sorry to say, cannot see the full force of your 

 rejoinder in regard to second head of continually augmented 

 sterility. You speak in this rejoinder, and in Par. 5, of 

 all the individuals becoming in some slight degree sterile 

 in certain districts : if you were to admit that by con- 

 tinued exposure to these same conditions the sterility would 

 inevitably increase, there would be no need of Natural 

 Selection. But I suspect that the sterility is not caused so 

 much by any particular conditions as by long habituation 

 to conditions of any kind. To speak according to pan- 

 genesis, the gemmules of hybrids are not injured, for 

 hybrids propagate freely by buds ; but their reproductive 

 organs are somehow affected, so that they cannot accumu- 

 late the proper gemmules, in nearly the same manner as 

 the reproductive organs of a pure species become affected 

 when exposed to unnatural conditions. 



This is a very ill- expressed and ill-written letter. Do 

 not answer it, unless the spirit urges you. Life is too 

 short for so long a discussion. We shall, I greatly fear, 

 never agree. 



