1897] PLACE OF ISOLATION IN ORGANIC EVOLUTION 245 



Physiological Isolation. This was first brought forward by 

 Professor G. J. Romanes. 1 By it is meant those cases where the 

 individuals of a species mix together during the breeding season, 

 but, for some reason or other, certain individuals are restrained from 

 having sexual intercourse with others. The simplest case is that of 

 a sexual reproduction which insures that each individual is isolated 

 from all others, and, consequently, any variations that may arise 

 are preserved, unless counteracted by reversion. Probably this is 

 the cause of the immense variety found among the Bacteria, Dia- 

 toms, Fungi, Radiolarians, and Foraminifera ; and perhaps it is the 

 reason why Bacteria are so readily modified when placed under new 

 conditions by cultivation. Self-fertilisation is nearly as efficient ; 

 but a cross may occasionally occur. Ferns and many other plants, 

 as well as many Coelenterates, are thus isolated and able to preserve 

 indifferent variations. 



Partial sterility with the parent form (the physiological selec- 

 tion of Professor Romanes) ; the selective association of Dr A. R. 

 Wallace ; and change in the season of flowering or of pairing, all 

 appear to be true causes of physiological isolation. I have lately 

 given an example of the process of species manufacture by the last 

 process in the case of some petrels on the Kermadec Islands. 2 Two 

 varieties of Aestrelata neglccta — the mutton-bird and winter mutton- 

 bird of the settlers — breed on the same island, but at different times 

 of the year. The first has the neck and breast, and sometimes the 

 whole under surface, gray ; while the winter mutton-bird has only 

 a gray band on the breast, the rest of the under surface being white. 

 Here physiological isolation is bringing about much the same result 

 as geographical isolation has done in the case of the albatrosses, for 

 — as with them — we must suppose that the change in the time of 

 pairing preceded the change of plumage. 



Sexual selection is better considered as a form of physiological 

 isolation than of natural selection, for there is no elimination of the 

 males ; they are not killed off, but can, after defeat, try again to 

 obtain a partner. Some males secure the females either by greater 

 strength or by superior weapons of offence, or by superior means of 

 capturing them, while others are selected or rejected by the females; 

 and in the case of birds, the latter mode of selection seems to ex- 

 plain the preservation of many beautiful variations in plumage. 

 Dr Wallace supposes that these beautiful variations in plumage 

 have been produced by the greater vigour of certain males, which is 

 probably true ; but no amount of vigour in the male would, by 

 itself, secure the preservation of these variations without isolation, 

 and this has been clue to sexual selection. It is possible that the 

 females select the males for their vigour and not for their beauty, 

 1 Journ. Linn. Soc. (Zool.), vol. xix., p. 350. 2 Proc. Zool. Soc, 1893, p. 753. 



