MR. Q-, J. EOMAJiTES ON PHYSIOLOGICAL SELECTION". 363 



ing an even increased degree of fertility towards others. For in 

 all cases, according to this theory, degrees of fertiKty between 

 allied forms are, so to speak, matters of accident ; and it is only 

 when variations in the direction of sterility v^'ith allied forms 

 (parental or otherwise) are sufficiently pronounced to prevent 

 intercrossing that the forms in question rise to specific rank. 

 Therefore, looking to the immense number of species, we might 

 expect that in some few cases where the allied forms are not also 

 contiguous, the variation in the reproductive system which 

 rendered one of the forms sterile with its parent form, should not 

 also have rendered it sterile with exotic forms, or even that it 

 should be more fertile with them than with itself. And this we 

 do occasionally find to be the case, as the following quotations 

 from Darwin will show. 



" Of his [Herbert] many important statements I will here give 

 only a single one as an example, namely, that " every ovule in a 

 pod of Crinum capense fertilised by C. revolutum produced a plant, 

 which I never saw to occur in a case of its natural fecundation." 

 So that here we have perfect, or even more than commonly perfect, 

 fertility in a first cross between two distinct species "*. 



Mr. Darwin then proceeds to give other and analogous cases as 

 having been well observed in Lohelia, Verhascum, and Passiflora ; 

 and then adds, "In the genus Sippeastrum, in Corydalis as 

 shown by Professor Hildebrand, in various orchids as shown by 

 Mr. Scott and Fritz Miiller, all the individuals are in this peculiar 

 condition. So that with some species, certain abnormal individuals, 

 and in other species all the individuals, can actually be hybridised 

 much more readily than they can be fertilised by pollen from the 

 same individual plant." 



Now, these and all other such facts go to prove that, notwith- 

 standing even a specific distinction, there may be a higher degree 

 of compatibility between the sexual elements of the different 

 forms than between the sexual elements of the same form ; and 

 this would show that in the matter of sexual compatibility more 

 depends upon the nature of the sexual elements than depends 

 upon the rest of the organism. In other words, we may here 

 regard the two distinct species as (physiologically considered) 

 extreme varieties, and thus we should have evidence of a higher 

 degree of fertility between these two extreme varietal forms than 



* ' Origin of Species,' ed. 6, p. 238 ; also see ' Variation,' vol. ii. pp. 143-4. 



