4 LIVERPOOL BIOLOGICAL SOCIETY. 
the most widely known, and which has excited most 
discussion during the last couple of years, is Dr. Romanes’ 
essay on “‘ Physiological Selection; an Additional Sug- 
gestion on the Origin of Species,’ read before the Linnean 
Society of London on the 6th May, 1886.* 
Dr. Romanes commences by stating that there are in 
his view three cardinal difficulties in the way of natural 
selection, considered as a theory of the origin of species. 
These are:—(1) That whereas domesticated varieties, the 
products of artificial selection, are generally fertile with 
one another, allied species, which we believe are formed 
in a similar manner by natural selection, are almost 
invariably sterile. There are exceptions in both cases, 
but still this great difference does undoubtedly exist. 
However, I think it can be accounted for without going 
beyond the influence of natural selection. In the first 
place the conditions surrounding domesticated varieties 
are more or less unnatural, and may well account for an 
increased fertility ; and in the second place, turning to 
variation in a state of nature, it is quite conceivable that 
the very features which characterize an incipient species, 
such as a difference in speed, or a peculiarity in colouring, 
may themselves tend.(by giving rise to isolation) to 
promote breeding between the individuals possessing such 
features and to put a barrier between them and the parent 
species. As Darwin has said,t ‘‘each newly formed variety 
would generally be at first local, as seems to be the 
common rule with varieties in a state of nature; so that 
similarly modified individuals would soon exist in a small 
body together, and would often breed together.” 
(2) Dr. Romanes’ next difficulty is the swamping effect 
which free intercrossing would have upon a variety or 
* Journ. Linn. Soc., Zool., vol. xix., p. 387. 
+ ‘Origin of Species,” 6 Ed., p. 72. 

