XI 



between them is a factor in the origin of species of which sufficient 

 account has not been taken. 



Romanes' precise position in relation to this contentious subject 

 may be stated very shortly. Other naturalists had shown that 

 isolation of part of a species, whether by geographical or other 

 external obstacles to sexual intercourse, inevitably led to the acquire- 

 ment by the plants or animals thus separated, of characters different 

 from those of the remainder, and eventually to the setting up of the 

 bar of reciprocal sterility between them. To this, without professing 

 to explain it, Romanes added the statement that the setting up of 

 this " physiological " barrier may be the primary event, i.e., that it 

 may come into existence between one part of a species and the rest 

 in the absence of any structural difference, or other external obstacle 

 to sexual union, and thus, having no cause itself, become the prime 

 cause of all other differences. He himself admitted that his " theory " 

 was but a statement of what he believed actually to happen, leaving 

 the explanation of it, with that of variation in general, to the " future 

 of physiology" ('Linn. Soc. Zool. Proc.,' vol. 19, p. 337). During 

 the following half-dozen years he spent much time and labour in 

 bringing together confirmatory instances, the results of which will no 

 doubt soon be published. The chief claim to consideration of this 

 theory consists in the prominence which it gives to what Romanes 

 calls " mutual sterility, with fertility inter se," as that which consti- 

 tutes the essence of species ; in other words, to the subordination of 

 all other specific distinctions to the one primary and dominant one 

 sterility to outsiders. Had he contented himself with showing that 

 whenever a variety is promoted to the rank of a species, that promo- 

 tion consists in the setting up of the " bar of sterility " that the pro- 

 tection of this barrier, however set up, is essential to the progressive 

 modification by natural selection of the species and that there are 

 reasons for believing that the functional disability may come into 

 existence in the absence of recognisable structural difference, no one 

 would have fallen into the mistake of supposing that it was his 

 intention to substitute a new doctrine for the Darwinian. 



As regards the question whether acquired characters are inherited, 

 it is sufficient to say that both the direct and the indirect evidence 

 seemed to Romanes to tend towards the affirmative. His physio- 

 logical training probably made him more alive than he would other- 

 wise have been to the difficulties which present themselves in all 

 those cases in which the apparatus required for the performance of a 

 complicated function must be in complete working order before it can 

 be of the slightest use to the organism, and consequently must have 

 arrived at perfection of development independently of the operation 

 of natural selection. The particular case in which he emphasises 

 this difficulty is that of the complicated automatic and perfectly 



