438 SECTIONAL TRANSACTIONS.— D. 



Dr. P. D. F. Murray. — Consequential evolution (12.5). 



The term ' Consequential Evolution ' is used when some change occurs 

 as a necessary result of some environmental or genotypic change and is 

 exerted via a developmental process which is itself unaltered. The con- 

 ception is discussed in the light of such instances as the effect of size 

 changes on proportions of parts (allometric growth in Titanotheres, horses, 

 etc.), the effects of variation in genes controlling the rates of biological 

 processes, and of alterations in gradient systems on developing patterns. 



General Discussion (12.20). 



Afternoon. 

 Joint Discussion (with Section K) on Mechanism of evolution {continued) 



(2.15). 

 Chairman : Prof. E. J. Salisbury, F.R.S. 



General theme of the papers. — Isolation and speciation. 



Dr. E. B. Worthington. — Geographical isolation with special refer- 

 ence to fresh waters (2.15). 



Evolution is controlled in part by external factors, and these can be 

 studied best where isolation, such as that afforded by islands or freshwaters, 

 limits the environment. In freshwaters the mixture of fauna and flora 

 is prevented by (a) physical barriers, such as watersheds and waterfalls, 

 (6) ecological barriers. The evolution of a habitat (e.g. a lake silting up or 

 undergoing chemical change) involves changes in fauna and flora. In a wet 

 climate such as England this is usually effected by the loss of some species 

 and the addition of others from outside, but in a dry climate where isolation 

 is more complete (e.g. parts of Africa) organic evolution may keep pace with 

 the evolution of habitats. This is demonstrated best in the tropics where 

 generations are passed through more quickly, and evolution is therefore 

 more rapid. In the presence of predators the intermediate stages in species 

 formation (when the organisms do not fit their environment) rarely survive ; 

 hence evolution is most rapid and most obvious where predators are rela- 

 tively unimportant. These principles are illustrated from British and 

 African waters. 



Dr. W. B. TuRRiLL. — Ecological isolation (2.35). 



The difficulty of limiting the phrase ' ecological isolation ' is discussed 

 and it is pointed out that ecological isolation is often exactly associated with 

 geographical and other lands of isolation. The evolutionary and taxo- 

 nomic significance of this association is considered. Examples of isolation 

 caused by the action of climatic, edaphic, and biotic factors respectively are 

 given. The importance of the complicated interaction of ecological factors 

 with one another is emphasised and it is shown that ' habit ' is a plant 

 character (or group of characters) frequently sifted out by natural selection. 



The taxonomic categories are primarily matters of scientific convenience 

 and the criteria used to delimit them need only be kept constant for any one 

 purpose. The species category is the most important for many purposes 

 and speciation is a concern of the taxonomist. Examples of speciation 

 correlated with ecological isolation (and sometimes probably initiated by 



