402 ME. G. J. EOMANES ON PHYSIOLOGICAL SELECTION. 



tinctions, agree in presenting the constant distinction in respect 

 of their reproductive systems. In other words, systematists, in 

 their classification of species, have always been engaged in un- 

 consciously tabulating the records of cases where overwhelming 

 intercrossing witli parent forms has been prevented ; and the only 

 way in which we can account for the now very frequent occur- 

 rence of sterility between allied species is by supposing that in 

 these cases it was this sterility which prevented the intercrossing, 

 or constituted the condition to these species being formed. It 

 serves still further to enforce this view of the case when we try 

 to imagine what would happen if the now existing sterility be- 

 tween all allied species which present it were suddenly removed. 

 In this case free intercrossing within the limits of each genus 

 would soon reduce all specific types living on common areas to 

 as small a number of species as there are now genera. But i£ 

 this is what would certainly be the result on all common areas i£ 

 the physiological conditions now existing were removed, must we 

 not conclude that it was owing to the fact of these conditions that 

 the now existing species arose ? 



Or, again, let us contrast the difference between natural species 

 and domesticated varieties. These, as we have seen, resemble each 

 other in every respect save in the one respect of mutual sterility. 

 Can we, therefore, doubt that this condition, so often as it 

 occurs, has played the same part in the evolution of natural 

 species as the prevention of intercrossing by artificial barriers 

 has played in the evolution of domesticated varieties ? Or can we 

 doubt tliat if intercrossing were in any other way prevented, 

 natural species would resemble domesticated varieties still more 

 closely in presenting well-marked differences of type without this 

 peculiar association with the barrier of sterility ? But if any one 

 should doubt this, we have only to point to the unquestionable fact, 

 that where intercrossing has been otherwise prevented — whether 

 by geographical barriers or by migration — such well-marked differ- 

 ences of type have arisen, though in these cases they are not 

 necessarily associated with the physiological barrier in question. 

 Therefore, when this barrier is present, how can it be reasonable 

 to doubt that its connection with the other differences of type is 

 a connection of casuality ? For does not this extraordinarily 

 general connection prove that it is only those cases of variation 

 in any other part of any organism which happen to have been 

 associated with the physiological barrier of sterility that have 



