3^7 
infertility with the rest of the species will serve as a 
protection against indiscriminate interbreeding should 
opportunity occur for that, through the invasion of some 
of the normal forms ; or through their encountering such 
on the outskirts of the locality in which they live. 
Mr. Meldona proposes to himself the question, “ Can 
physiological selection work independently of Natural 
Selection ? ” and he decides that it cannot. 
“ Let us suppose, for the sake of argument, that among the indi 
viduals of a species there arise certain varieties which are fertile 
inter se, but sterile with the parent form. There would thus arise a 
new race which could not be swamped by intercrossing with the 
predominant form, and the one species would practically be resolved 
into two—the parent form being still in the ascendency as regards 
numbers. But the competition is always most severe between the 
most closely related forms, and unless the new form (arising by 
inter-racial amixia) possessed some distinct advantage over the old 
one, it would as surely be exterminated by the overwhelming majority 
of the parent type, as it would be by intercrossing in the absence of 
amixia. Physiological selection thus appears to me to be as sub 
ordinate to natural selection as sexual selection, correlated variability, 
the law of homology, or any other of the Darwinian factors. The 
expression used by Mr. Romanes for his new factor—the ‘ Segrega 
tion of the Fit’—seems to imply fitness for something, presumably 
for the conditions of life, and if the survival of the ‘fit’ race is 
determined by Natural Selection, then I venture to think that 
Natural Selection must still be regarded as the theory of the origin 
of species and as something more than a theory of the origin of 
adaptations.”—(Nature. vol. xxxiv., pp. 384-5.) 
This argument is based upon a belief in the intenser 
struggle for existence between closely allied forms and 
upon the idea of utility as the condition of survival in 
the case of such struggle. But in some cases, at any rate, 
closely allied species do not compete with one another 
in an internecine strife. Sometimes they live together 
harmoniously. And if we look to local or climatic varia 
tions, we shall find that in some cases, at any rate, the