10 LIVERPOOL BIOLOGICAL SOCIETY. 
“Suppose the variation in the reproductive system is such that the season 
of flowering or of pairing becomes either advanced or retarded. Whether this 
variation be, as we say, spontaneous, or due to any change of food, habitat, 
climate, &c., does not signify. The only point we need here attend to is that 
some individuals, living on the same geographical area as the rest of their 
species, have varied in their reproductive systems, so that they can only 
propagate with each other. They are thus perfectly fertile inter se, while 
absolutely sterile with all the other members of their species. This particular 
variation being communicated by inheritance to their progeny, there would 
soon arise on the same area, or, if we like, on closely contiguous areas, two 
varieties of the same species, each perfectly fertile within its own limits, while 
absolutely sterile with one another. That is to say, there has arisen between 
these two varieties a barrier to intercrossing which is quite as effectual as a 
thousand miles of ocean; the only difference is that the barrier, instead of 
being geographical, is physiological.” 
Now although I have been arguing hitherto that natural 
selection is sufficient to account for the production of new 
species, I do not mean to say that physiological selection 
does not occur also. On the contrary, it seems to me that 
Dr. Romanes has made out a very good case in favour of 
the possibility of physiological selection occurring ; and it 
may therefore be placed along with certain other processes, 
after natural selection, as an additional but subordinate 
factor in the evolution of species. In fact, as Darwin 
himself recognised, there are probably several distinct pro- 
cesses at work besides natural selection in modifying and 
diversifying the organic types; and Mr. Herbert Spencer 
has lately done good service in calling the attention of 
evolutionists to these somewhat neglected secondary 
factors.* 
The effects of use and disuse in modifying some parts of 
the animal structure are very liable to be confused with 
and classed under the action of natural selection, but in 
such a case as the well-marked degeneration in the entire 
* The Factors of Organic Evolution, Nineteenth Century for 1886, pp. 570 
and 749, 

