SECTIONAL TRANSACTIONS.— D. 439 



it) are given, especially from the flora of the Balkan Peninsula. It is con- 

 cluded that ecological isolation is (and especially has been) one of the factors 

 involved in speciation in the wild, but that it is often broken down by man's 

 interference. The importance of the study of wild floras and faunas, from 

 all standpoints, thus becomes evident, if the processes not only of speciation 

 but of evolution in general are to be understood. 



Dr. W. H. Thorpe. — Physiological isolation (2.55). 



Dr. C. D. Darlington. — Genetic isolation (3.15). 



John Ray described a species as a group breeding true within its own 

 limits. Modern genetics show that this is a definition valid in theory and 

 practice. It means that the origin of two species from one must depend on 

 a barrier which effectively prevents inter-breeding in nature. We now 

 know that such barrriers are broadly of two kinds, external and internal. 

 The internal barriers act physiologically or mechanically but are determiried 

 genetically. Genetic isolation may act in plants by the pollen growing 

 more quickly on styles having certain similar genes than on styles having 

 those genes in a mutant form. It may act in an analogous way in animals 

 through the mating instincts or capabilities. The building up of such 

 differential systems cannot be achieved in one step by a gene mutation. In 

 nature it seems that its origin usually depends on the action of a group of 

 genes. Such a group can be held together only when associated with a 

 sudden change in the arrangement of genes. Changes in arrangement alone 

 can produce genetic isolation, without the differential action of genes, by 

 causing sterihty of the hybrid. Since genetic isolation will frequently give 

 rise to geographical or ecological isolation in nature and they will regularly 

 give rise to genetic isolation, the order of events cannot always be inferred. 

 But where genetic isolation alone is concerned, the several steps establishing 

 it have been disentangled in particular instances by gene and chromosome 

 analysis, and placed in the chronological order indispensable to their evolu- 

 tionary development. 



Dr. D. G. Catcheside. — Chromosomal isolation (3'.35). 



Chromosomal isolation may operate by preventing the formation of a 

 hybrid or, more usually, by preventing in a hybrid the normal exchange of 

 genetic material between parts of chromosomes, whole chromosomes or sets 

 of chromosomes. The former frequently occurs in flowering plants where- 

 ever there is an upset in the normal one-to-two relation between the chromo- 

 some numbers of pollen and of style or an upset in the two-to-three relation 

 between embryo and endosperm. The latter method operates at meiosis 

 wherever there is a numerical or a structural difference, such as an in- 

 version or an interchange, between the chromosomes of the gametes pro- 

 ducing the individual. There is a reduction in fertility, which is particularly 

 at the expense of cross-over gametes in structural hybrids. Simple examples 

 illustrating these principles are described. 



Discussion (3.55). Opened by Dr. S. C. Harland. 



Saturday, August 20. 



Excursion to Wicken Fen and the Break country. 



