390 
JOURNAL OF THE PLYMOUTH INSTITUTION, 
Darwin in assigning a greater importance to the influence of 
isolation upon the origin of species. 
The fact that the occurrence of violent and sudden changes in the 
physical aspect of the world is necessary, in order to account for 
the great influence Dixon considers isolation has had in originating 
species, was urged to be a fatal objection to the theory, and Dar- 
win's statement, that "natural selection has been the main but 
not the exclusive means of modification," was considered to be the 
most correct expression of our present knowledge of the subject. 
The lecturer then passed on to consider Mr. Romanes' paper 
(Linnean Soc, May, 1886) on "Physiological Selection, an Addi- 
tional Suggestion on the Origin of Species." 
Mr. Romanes' statement, that natural selection is not a theory 
of the origin of species at all, but a theory of the cumulative 
development of adaptation, was considered, and the fact pointed 
out that natural selection does not originate the variations which 
constitute a species, and is not therefore the cause, in that sense, 
of their origin ; but that it consolidates certain of the innumerable 
variations, which are brought about by various agencies, renders 
them of imperative necessity, and by this means forms a species. 
Romanes was shown to maintain that "the principle of the 
prevention of intercrossing with parent forms, or the evolution of 
species by independent variation " is the most important influence 
in the formation of species; and that on account of (1) the very 
general occurrence of sterility between closely allied species, and 
(2) the fact that closely allied species are not always separated by 
geographical barriers, he does not consider isolation a sufficient 
reason to account for the phenomenon, and suggests an additional 
factor in the formation of species, and calls that factor "Physio- 
logical selection." He urges that by means of it species are 
formed from varieties, which would otherwise be absorbed, and 
that it is only by the help of physiological selection that natural 
selection can work at all. 
The theory was discussed at some length, together with the 
various criticisms the publication of the theory had called forth. 
It was pointed out that the term physiological selection, as 
applied to the phenomena detailed by Romanes, is not a distinctive 
term, the definition of the term physiology showing that it deals 
only with the actions of living beings or their surroundings, and 
the effect of the surroundings on the living beings. 
