3io 



SCIENCE-GOSSIP. 



enforcing his own opinion. Wallace's arguments 

 against sexual selection show us conclusively its 

 absurdity. His contention that natural selection is 

 able to effect what Darwin attributed to sexual, fails 

 to carry conviction. Weismann is infinitely more 

 successful in proving that there is no transmission 

 of acquired characters, than in showing how 

 natural selection can do the work without it. 

 Dr. Romanes carries us irresistibly with him when 

 he proves that natural selection alone cannot 

 produce divergent evolution. When he comes to 

 his theory of physiological selection, which is to 

 supplement it, we cannot follow him without 

 protest. 



The book begins by insisting on the extreme 

 importance of isolation as a factor in evolution. 

 Every naturalist has more or less recognised its 

 importance, but the author thinks only he 

 and Mr. Gulick have realised how important it 

 really is. The principle of isolation is deeper and 

 of larger influence than even natural selection 

 itself. Heredity, variation and isolation form the 

 three pillars upon which the whole superstructure 

 of organic evolution is reared. By isolation is here 

 meant anything which prevents intercrossing be- 

 tween one section of a species and the rest of 

 that species. Dr. Romanes believed that " in the 

 presence of free intercrossing natural selection 

 would be powerless to effect divergent evolution." 



" The point then," he insists, " which in the 

 first instance must be firmly fastened in our minds 

 is this : so long as there is free intercrossing, 

 heredity cancels variability and makes in favour of 

 fixity of type. Only when assisted by some form 

 of disc iminate isolation, which determines the 

 exclusive breeding of like with like, can heredity 

 make in favour of change of type, or lead to what 

 we understand by organic evolution." 



The difficulty here emphasised is one which has 

 been felt and dwelt on by most critics of Darwin- 

 ism, and has been replied to — but never, we 

 believe, satisfactorily — by leading Darwinians. 

 What is wanted, certainly, is something which 

 determines the " exclusive breeding of like with 

 like." We must, however, surely admit that every 

 evolutionist has acknowledged the need for 

 isolation, and the breeding of like with like. As 

 regards this, indeed, it appears that the difference 

 between Dr. Romanes and others is partly verbal 

 and partly a question of the degree in which the 

 various factors concerned have entered into 

 evolution. How far, then, we may inquire, does 

 Dr. Romanes' new view render the "exclusive 

 breeding of like with like" more probable? 

 Isolation in general is first of all treated, and 

 two kinds are distinguished. Thus, if a shepherd 

 divides his flock by placing all the white sheep in 

 one field and all the black in another, or if in a 

 certain species a change of instinct in a part of the 



same determines migration to a new area, the 

 isolation is discriminate. If, on the other hand, 

 a shepherd divides his sheep without any regard 

 to their characters, or if geological subsidence 

 separates a species into two parts, we have 

 indiscriminate isolation. Both these forms of 

 isolation are potential in the formation of new 

 species. 



The reader who has got thus far in the argument 

 wonders what has become of natural selection, and 

 begins to feel that the charge sometimes brought 

 against the author of having substituted physio- 

 logical for natural selection must be true. But 

 on page 9 we learn that natural selection is to be 

 looked upon as a form of isolation, insomuch as 

 it kills off one part of a species, and thus isolates 

 another. 



We now come to physiological selection, another 

 form of isolation, and obviously considered by the 

 author to be the most important of all. Let us 

 inquire, then, what physiological selection is, and 

 to what extent, if any, it supplies the universal 

 desideratum of evolutionists, a something which de- 

 termines the "exclusive breeding of like with like." 

 It is defined as "sexual incompatibility — either 

 partial or absolute — between different taxonomic 

 groups." Again, it is described as " that form of 

 isolation which arises in consequence of mutual 

 infertility between the members of any group of 

 organisms and those of all other similarly isolated 

 groups occupying simultaneously the same area." 

 It interposes " the bar of sterility between two 

 sections of a previously uniform species ; and by 

 thus isolating the two sections one from another, 

 starts each upon a subsequently independent course 

 of divergent evolution." As the starting-point of 

 the theory, the fact that " some degree of infertility 

 is not unusual as between different varieties of the 

 same species" is brought forward. Thus we gather 

 that the beginning of physiological selection is some 

 degree of infertility — some diminished fertility — between 

 individuals of a species. Does this theory, then, 

 give us anything to determine " the exclusive 

 breeding of like with like " ? We must first ask, 

 is such cross-infertility necessarily accompanied 

 by disinclination to breed together ? As far as 

 we can gather, Dr. Romanes does not contend 

 that the infertility of certain individuals with the 

 parent stock necessitates an inclination to breed 

 together rather than with the latter. Nor is there any 

 evidence to show that this is so. Thus Dr. Romanes' 

 theory is either inefficient or superfluous. For 

 if infertility does not happen to be accompanied by 

 inclination to breed together it will be eliminated, 

 like any other small variation, by intercrossing ; 

 and if it chance to be accompanied by the same it 

 is not required, for as long as the individuals keep 

 separate infertility or increased fertility with the 

 parent stock is quite immaterial. Thus, while it 





