358 SECTIONAL TRANSACTIONS.— D. 



SECTION D.— ZOOLOGY. 



Thursday, September 10. 



Presidential Address by Dr. J. S. Huxley on Natural Selection and 

 evolutionary progress (io.o). 



Discussion on Selection (n.o). 



Dr. Timofeef-Ressovsky.— Introduction. 



Prof. G. D. Hale Carpenter, M.B.E. — Entomology and Natural 

 Selection (11.25). 



The following are difficult to explain without Natural Selection : 



(1) Mimicry in Uganda of species of the Acraeine genus of butterflies 

 Bematistes ( = Planema) by forms of one species of Nymphaline Pseudacraea 

 eurytus. Not only are local species of Bematistes mimicked by forms of 

 eurytus numerically proportional to their models, but if the protective 

 influence of Bematistes is diminished by their scarcity the forms of eurytus 

 depart from strict resemblance and intermediates abound. (2) Experiments 

 with a monkey under natural conditions showed that out of 143 species of 

 insects with conspicuous (aposematic) coloration 120 were distasteful, and 

 out of 101 with concealing (procryptic) coloration 83 were edible. Lycid 

 beetles, much mimicked in all tropics, were not eaten even under conditions 

 of considerable hunger : they are typically aposematic. Procryptic weevils 

 and Mantidae were greedily eaten. (3) Adaptations of flowers and insects 

 for mutual benefit . A very peculiar relationship exists between an Australian 

 orchid and an Ichneumon, the male of which, deceived by odour like that of 

 its female, enters a flower backwards and while depositing its spermatozoa 

 on the stigma causes the pollinia to adhere to its abdomen and thus with- 

 draws them. 



Dr. C. Gordon. — Evidence for natural selection from the genetic 

 analysis of free- living populations of Drosophila (11.40). 



The frequency of heterozygosis in free-living populations of Drosophila 

 of both sex-linked and autosomal mutants can be determined by appropriate 

 methods of inbreeding. The great bulk of evidence shows a considerable 

 frequency of autosomal recessive mutants and an absence of sex-linked 

 recessives, and dominants. It is clear from the data that sex-linked re- 

 cessives have been eliminated by natural selection, unless the assumption 

 is made that the mutation rate of autosomal genes is vastly greater than that 

 of sex-linked genes, which is not the case. 



Another line of evidence which should be developed further is the direct 

 method of releasing a balanced laboratory population whose genetic con- 

 stitution is known and estimating the frequency of heterozygosis of the 

 mutant type from time to time. A preliminary experiment which I under- 

 took in 1934 showed that this is possible. The frequency of a mutant 

 gene ebony declined from o -5 to o • 1 10 ± 0*03 in the course of one summer. 

 At the present time I am investigating various areas as to their suitability 

 for this type of work. 



