350 MR. G. J. EOMANES ON PHYSIOLOGICAL SELECTION. 



am collecting, of this general fact must be left to constitute the 

 subject of a future publication*. 



Physiological Selection, oe Segregation of the Pit. 



Were it not for the very general occurrence of some degree 

 of sterility between allied species, and were it not also for the 

 fact that closely allied species are not always, or even generally, 

 separated from one another by geographical barriers f, one might 

 reasonably be disposed to attribute all cases of species-formation 

 by independent variability to the prevention of intercrossing by 

 geographical barriers, and by migration. But it is evident that 

 these two facts can no more be explained by the influence of 

 geographical barriers, or by migration, than they can be by the 

 influence of natural selection. It is therefore the object of the 

 present paper to suggest an additional factor in the formation of 

 specific types by independent variability, and one which appears 

 to me fully competent to explain both the general facts just 

 mentioned. 



Of all parts of those variable beings which we call organisms, 

 the most variable is the reproductive system. It is needless for 

 me to remind any reader of Mr. Darwin's works what a mass of 

 evidence he has accumulated, showing the extreme sensitiveness 

 of the reproductive system to small changes in the conditions of 

 life. 



The consequent variations may occur either in the direction of 

 increased fertility, as with our domesticated varieties, or in that 

 of sterility in all degrees, as with wild species when confined. So 

 extreme is the sensitiveness of the reproductive system in these 

 respects — or, in other words, so liable is this system to vary — 



* So far as I am aware, the first writer who insisted on the great importance 

 of the prevention of intercrossing iu the evokition of species, both by isolation 

 and migration, wa,s Moritz Wagner. Since then Wallace, Weismami, and 

 others, as also Darwin himself, have in lesser degrees recognized this factor. 

 The most recent contribution to the subject is bj a Fellow of this Society, 

 Mr. Charles Dixon, whose work on ' Evolution without Natural Selection ' 

 presents a large and admirable body of facts, showing the important part 

 which the prevention of intercrossing has played in the evolution of species 

 among Birds. But I cannot find that any previous writer has alluded to the 

 principle which it is the object of the present paper to enunciate, and which is 

 explained in the succeeding paragraphs. 



t As Mr. Wallace observes, allied species usually occupy contiguous areas, 

 which more often than not are likewise continuous. 



