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 tc 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. Two 
varieties of Aestrelata neglecta—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 due 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. 850. 2 Proc. Zool. Soc., 1893, p. 758. 
