ME. a. J. EOMAJSTES ON PHYSIOLOGICAL SELECTION. 37l 



allied forms, with continued fertility within the varietal form. 

 Wow, this state of matters as between allied species is merely an 

 intensification, or a further development, of that which physiolo- 

 gical selection supposes to obtain between the physiological varie- 

 ties, where the variation is from the first in the direction just 

 mentioned. Tliat this initial variation should afterwards become 

 intensified by the practical separation of the two varieties, so that 

 what began as a varietal difference ends as a specific difference, is 

 no more than we should expect. For from the first the variation 

 was one specially affecting the reproductive system in the special 

 way required ; intercrossing with the parent form was from the 

 first precluded in a degree proportional to the amount of the 

 variation. The species was thus from the first divided into two 

 physiological parts, each of which then entered upon an inde- 

 pendent course of genetic history ; the principle of continued 

 variation along the same lines would tend to increase the original 

 separation ; the new variety, therefore, besides having been thus 

 started with a tendency, and a probable increasing tendency, to 

 a physiological separation from its parent stock, must afterwards 

 have become exposed to all or any such modifying causes as are 

 found to produce a similar separation in a portion of a species 

 when started on an independent course of history by migration 

 or by geographical isolation. 



Lastly, over and above all these considerations, there remains 

 one of much importance, not only to the present division of my 

 argument, but to my theory as a whole. !For Mr. Darwin has 

 furnished exceedingly good reasons for entertaining his own 

 view that this is " one of the causes of ordinary variability ; 

 namely, that the reproductive system, from being eminently 

 sensitive to changed conditions of life, fails under these circum- 

 stances to perform its proper function of producing offspring 

 closely similar in all respects to the parent form"*. JSTow, if 

 this view is well founded — and, as I have said, Mr. Darwin's 

 arguments in favour of it are most cogent — it obviously has most 

 important bearings on the present theory ; for it implies tliat 

 whenever the reproductive system undergoes a variation on its 

 own account, whether this be due to extrinsic or intrinsic causes 

 it is apt to induce variations in other parts of progeny. Hence 

 prevention of intercrossing by the physiological barrier of re- 

 productive or primary variation is so far more likely to be followed 

 * ' Origin of Species,' ed. 6, p. 260. 



29* 



