68 SECTIONAL ADDRESSES 
the lower as well as of the higher forms of animal life, a point in which 
animals differ from plants and a point which has usually been overlooked 
in discussing the relationship of animals to their environment. 
What I have called choice appears to play an even more important part 
than in mere localisation and distribution of species, and this is brought 
out in the observations that have been made upon biological races. Most 
of the work which has been done in this connection has been upon vege- 
tarian species, but such races are probably much more widely spread than 
has been recognised. Where a vegetarian species has several food plants, 
it has been found possible, in some cases, to induce a preference for one 
of these by keeping a number of generations on that plant so that, ulti- 
mately, a race is formed which restricts itself to that food plant. Although 
in most cases known in nature, no morphological changes have occurred, 
in a few cases changes of habit have appeared and, in a very few, certain 
other changes have been recorded. Thus, the ‘ Railroad fly,’ 1” an apple 
pest of North America, has a larger race on apple and a smaller one on 
blueberry. Cameron ® noted a colour variety of the Belladonna Leaf- 
mining fly, known as ‘var. betz,’ definitely associated with the race 
found on chenopodiaceous plants and absent from the solanaceous race. 
Again, Nuttall,!8 who showed that the head and body lice of man are 
biological races of one species, stated that, although identical in all 
essential points of structure, they differ in habit, in feeding habit, in size 
and in the thickness and length of antenne and length of legs, and he 
explained these differences as due to darkness (inducing longer and finer 
antennz and longer legs in the body form), feeding habit (inducing a 
larger size in the body form, which takes large meals at intervals, the 
head form feeding frequently). Thus he associated the changes with 
physiological influences and, as the two races are on the same host, their 
origin must have been connected with choice on the part of individuals. 
Physiological effects are far-reaching and, although we have as yet no 
good evidence from these biological races that new species have arisen 
in this way, we have abundant evidence of the effects of function upon 
structure. Both Darwin and Wallace stressed the importance of use- 
inheritance, that is, the inheritance of functionally-produced modifica- 
tions, so that it seems possible that physiological change, induced by 
surrounding conditions, may have had far-reaching effects. 
A study of any group shows that species differ in their relationships to 
one another; some form clusters and are difficult to distinguish, while 
others stand apart. So far as the water-beetles are concerned, the 
clustered species are not usually members of the same community or 
sometimes even of the same district. Although there is no evidence 
that they are recently separated species, the fact that they occupy different 
habitats, and that the differences are small, at least suggests that they 
may have originated from biological races. In this country we have many 
examples of these clusters. ‘The two species Agabus affinis and unguicularts 
17 Rhagoletis pomonella, Walsh. 
18 “The Systematic Position and Iconography of Pediculus humanus and 
Phthirus,’ Parasitology, 1919. 
