24 
James Small. 
Physiological Differentiation and Restriction of Area. Con 
sidering the list given by Willis (78, p. 206) of causes which may 
modify the application of the law of Age and Area, and considering 
also Bentham’s remarks quoted in Section A, we can trace an 
underlying principle which may be expressed as above. In a 
paper read before Section K of the British Association in 1916 (67) 
the writer applied the law of Age and Area to absolute age and total 
area and indicated the relation of physiological differentiation to 
restriction of area somewhat as follows : “ We find in the Compositae 
that the morphologically higher or more differentiated groups are 
well developed in the same regions as their morphologically lower 
or less differentiated ancestors. As these groups of ancestors and 
descendants occupy the same areas and show their maximum 
concentrations in these areas it is evident that the survival of the 
fittest in so far as it applies to morphological development has 
notexercised any very rigorous action, otherwise the ancestral groups 
would have been eliminated by their more highly developed offspring.” 
“ On the other hand we know nothing very definite about the 
physiological differentiation of these groups and from analogy it 
would seem probable that within the limits of any particular family 
a morphologically primitive plant would be physiologically primitive 
and that a morphologically advanced plant would be physiologically 
highly differentiated also. The result of this would be that, although 
the higher forms would be more successful in those areas peculiarly 
well suited to their physiological constitution, this advantage would 
be more or less counterbalanced by the greater range, or greater 
choice of habitat, which would be enjoyed by the lower forms on 
account of their lack of special physiological differentiation. This 
compensation of the advantages of adaptation to one habitat by the 
power of living in many different habitats seems the most probable 
explanation of the continued existence in such profusion of Senecio, 
the primitive genus of the family.” 
“ Of course, morphologically reduced forms must be carefully 
distinguished from the primitive forms, as these reduced forms 
would probably have a relatively highly differentiated physiological 
constitution.” It should be further noted that a genus with an 
unstable constitution, (i.e., abnormally susceptible to environmental 
conditions and liable to spontaneous mutations) will probably not 
show a species covering the range of the genus, while a more stable 
genus may have this characteristic (cp. 75, p. 336), 
Guppy in the following year independently expressed the same 
