The Wallace-Darwin Correspondence 



Selection, of two already sterile species seems to me best 

 brought home by considering an actual case. The cow- 

 slip and primrose are moderately sterile, yet occasionally 

 produce 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 seeds) than usual, 

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

 the ultimate exclusion of the present primrose and cow- 

 slip ? I cannot. 



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

 your rejoinder in regard to the 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 continued 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 pangenesis, the gemmules of hybrids are not 

 injured, for hybrids propagate freely by buds ; but their 

 reproductive organs are somehow affected, so that they 

 cannot accumulate 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. — My dear Wallace, most sincerely yours, 



Ch. Darwin. 

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