392 ME. G. J. ROMANES OK PHYSIO LOGIC AL SELECTION. 



reproductive system, seeing that this system is the most sensitive 

 to small changes in the conditions of life, and that the greater 

 the number of individuals composing a specific type, the more 

 certainty there is of some of them encountering such changes. 

 Now, of all the variations going on in all parts of the organic type, 

 those which occur in the reproductive system of the kind required 

 by physiological selection are most likely to be preserved, seeing 

 that all other variations are likely to be swamped by free inter- 

 crossing. Hence, the richness of dominant species in varieties is, 

 I believe, mainly due to the greater opportunity which such 

 species afford of some degree of sterility arising between its 

 constituent members. 



Here is another general fact, also first noticed by Darwin, 

 and one which he experiences some difficulty in explaining on 

 the theory of natural selection. He says : — " In travelling from 

 north to south over a continent, we generally meet at successive 

 intervals with closely-allied or representative species, evidently 

 filling the same place in the economy of the land. These 

 representative species often meet and interlock, and as one 

 becomes rarer and rarer, the other becomes more and more 

 frequent, till the one replaces the other. But if we compare 

 these species where they intermingle, they are generally as 

 absolutely distinct from each other in every detail of structure as 



are specimens taken from the metropolis of each In the 



intermediate region, having intermediate conditions of life, why 

 do we not now find closely-linking intermediate varieties ? This 

 difficulty for a long time quite confounded me. But I think it 

 can in large part be explained "*. 



This explanation is that, as " the. neutral territory between 

 two representative species is generally narrow in comparison 

 with the territory proper to each, .... and as varieties do not 

 essentially differ from species, the same rule will probably apply 

 to both ; and, therefore, if we take a varying species inhabiting 

 a very large area, we shall have to adapt two varieties to two 

 large areas, and a third variety to a narrow intermediate zone." 

 It is hence argued that this third or intermediate variety, on 

 account of its existing in lesser numbers, will probably be soon 

 overrun and exterminated by the larger populations on either side 

 of it. But surely this argument overlooks one all-important fact, 

 namely, that varieties <?o " essentially differ from species" in 

 * ' Origin of Species,' eel. 6, pp. 134-135, 



