42 Darwin, and after Darwin 



The following is a brief outline sketch of this 

 theory ^. 



Of all parts of those variable objects which we 

 call organisms, the most variable is the reproductive 

 system ; and the variations may carry with them func- 

 tional changes, which may be either in the direction 

 of increased or of diminished fertility. Consequently 

 variations in the way of greater or less fertility fre- 

 quently take place, both in plants and animals ; and 

 probably, if we had adequate means of observing this 

 point, we should find that there is no one variation 

 more common. But of course where infertility arises 

 — whether as a result of changed conditions of life, or, 

 as we say, spontaneously — it immediately becomes 

 extinguished, seeing that the individuals which it 

 affects are less able (if able at all) to propagate and 

 to hand on the variation. If, however, the variant, 

 while showing some degree of infertility with the parent 

 form, continues to be as fertile as before when mated 

 with similar variants, under these circumstances there 

 is no reason why such differential fertility should not 

 be perpetuated. 



Stated in another form this suggestion enables us 

 to regard many, if not most, species as the records of 

 variations in the reproductive systems of their ancestors. 

 When variations of a non-useful kind occur in any 

 of the other systems or parts of organisms, they are, 

 as a rule, immediately extinguished by intercrossing. 

 But whenever they arise in the reproductive system 

 in the way here suggested, they tend to be preserved 

 as new natural varieties, or incipient species. At 

 first the difference would only be in respect of the 

 ' See Nineteenth Century, January, 1887, pp. 61, 6a. 



