400 SECTIONAL TRANSACTIONS.—D. 
group living in different regions have attained their characteristic form independently, 
by parallel or convergent evolution. Therefore a group under review from a 
zoogeographical standpoint should be critically examined as to whether its origin was 
probably monophyletic or polyphyletic. 
What criteria are to be used ? Comments are made on this difficulty and a 
distinction is drawn between resemblances due to the independent adaptation of 
organs to perform a similar function, and cases of convergence which present no 
evidence of functional adaptation. Some examples are discussed and illustrated 
with diagrams: (i) instances in which the modifications characterising a group 
affect only a single organ or several closely correlated organs, and have almost certainly 
arisen independently from different parent-stems, while the animals so modified are 
classed together in one group mainly for convenience (polyphyletic origin) ; in these 
cases the modifications may be a direct response to environment and, since the 
modified forms arise independently in different countries, conclusions of the kind 
indicated above cannot be based on their geographical range ; (ii) an example is given 
of modifications affecting several different parts of the organism, not apparently 
correlated nor arising as a direct result of environmental conditions, and it is contended 
that the group of species so modified had a monophyletic origin, and therefore their 
presence in widely remote lands is important from a zoogeographical standpoint. 
Emphasis is laid on the need for observation in the field and the application of experi- 
mental methods, whenever possible, so that direct evidence may be gained as to 
which classes of characters arise by convergence and which have a deeper-seated, 
phylogenetic, origin. 
Tuesday, September 29. 
SyMPosIuM ON VARIATION AND GENETICS :— 
Prof. J. W. Hestor Harrison.—Recent Work on Induced Mutations. 
Dr. W. R. THompson.—The Species Concept in relation to the Problem 
of Evolution. 
Dr. W. H. THorpr.—Bvrological Races in relation to the Problem of 
Evolution. 
A biological race is a sub-specific group of individuals separated from other members 
of the same morphological species by definite and constant biological differences ; 
structural and pigmentary differences being either completely absent or else very 
slight and inconstant. The characters by which biological races are distinguished 
usually concern the food, egg laying and mating preferences, and seasonal distribution. 
Thus, whereas a geographical subspecies is characterised by differences of average 
size or pigmentation and is isolated by geographical barriers, a biological race is 
characterised primarily by biological characters and is isolated by biological differences 
such as food preferences and mating preferences. 
Biological races are known to exist in most of the main invertebrate groups. They 
appear to be most highly developed in the Insecta, Arachnida, and the Nemathelminthes. 
There is also evidence of their existence in Calenterata, Porifera, and Protozoa. In 
the two latter groups, where morphological characters are often indefinite and un- 
reliable, the concept of a species fades insensibly into that of a biological race. This, 
of course, does not imply that the morphological characters of a species, even in the 
higher groups, are any more fundamental or important than the biological ; they are 
accentuated merely for the sake of convenience in systematic work. 
With regard to insects the host plant preferences which provide the basis for most 
biological races may be either germinally fixed or may be merely ‘ memory reactions.’ 
The latter are renewed in each generation owing to the preference of an adult insect 
for oviposition on the particular food plant on which it was nourished as a larva— 
the so-called ‘ Host Selection Principle.’ 
In the case of those which are germinally fixed the probability that such biological 
characters may be of survival value is obvious. But even where the characters are 
of no direct value or where they are not germinally fixed they may well be of evolutionary 
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