ME. Q. J. EOMANES ON PHYSIOLOGICAL SELECTION. 369 



the same genus, these opposite cases may occur"*. Now, on 

 the supposition that sterility between species is always or gene- 

 rally caused by the indirect influence on the reproductive system 

 of changes taking place in other parts of the organism, these 

 facts are unintelligible — being, indeed, as a mere matter of logic, 

 contradictory of the supposition. 



Fourthly, it is surely a most significant fact that, as Mr. 

 Darwin observes, " independently of the question of fertility, in 

 all other respects there is the closest general resemblance between 

 hybrids and mongrels " f. For this fact implies that natural 

 selection and artificial selection run perfectly parallel in all other 

 respects, save in the one respect of reacting on the reproductive 

 system, where, according to the views against which I am 

 arguing, they must be regarded as difi'ering, not only constantly, 

 but also profoundly. 



Fifthly, and lastly, Darwin further observes that " the primary 

 cause of the sterility of crossed species (as compared with 

 crossed varieties) is confined to differences in their sexual 

 elements " J. Now this assuredly proves that the primary 

 specific distinction is one with which the organism as a whole is 

 not concerned ; this primary distinction is, so to speak, a local 

 variation in the organism, which has to do only with the repro- 

 ductive system, and which therefore need not necessarily be in 

 all, or even in most, cases an incidental result of minute varia- 

 tions going on elsewhere. 



In view of these several considerations, it appears to me 

 perfectly plain that the smaller organic changes which alone 

 are concerned in specific distinctions are not always, or 

 even generally, adequate to react on the reproductive system 



* He also adds ; — " No one has been able to point out what kind or amount 

 of difference in any recognizable character is suiScient to prevent two species 

 crossing. It can be shown that plants most widely different in habit and 

 general appearance, and having strongly marked differences in every part of 

 the flower, even in the pollen, in the fruit, and in the cotyledons, can be crossed. 

 Annual and perennial plants, deciduous and evergreen trees, plants inhabiting 

 different stations and fitted for extremely different climates, can often be 

 crossed with ease." And, after considering the further case of reciprocal 

 crosses, he expresses the general conclusion : " Such cases are highly important, 

 for they prove that the capacity in any two species to cross is often completely 

 independent of their systematic affinity, that is of any difference in their 

 structure or constitution, excepting in their reproductive systems." (' Origin of 

 Species,' ed. 6, p. 243.) 



t ' Origin of Species,' where the general fact is proved beyond question. 



J Log. cit. This fact, also, is proved beyond the possibility of question. 

 LINN. JOUKN. — ZOOLOGT, YOL. XES. 29 



