D.—ZOOLOGY. 9i) 



It is possible that a stud}' of the history of a single-year class of a 

 population of fish living in such an isolated environment as a lake, would 

 yield very valuable information on the adaptive significance of some 

 determinable variations. It is unfortunate that the extensive migrations 

 of herring in the West European waters render the data accumulated by 

 Fisheries Investigators unsuitable for the purpose. 



The extraordinary lack of evidence to show that the incidence of 

 death under natural conditions is controlled by small differences of the 

 kind which separate species from one another or, what is the same thing 

 from an observational point of view, by physiological differences correlated 

 with such structural features, renders it difficult to appeal to. natural 

 selection as the main or indeed an important factor in bringing about the 

 evolutionary changes which we know to have occurred. 



It may be important, it may indeed be the principle which overrides all 

 others ; but at present its real existence as a phenomenon rests on an 

 extremely slender basis. 



The extreme difficulty of obtaining the necessary data for any quanti- 

 tative estimation of the efficiency of natural selection makes it seem 

 probable that this theory will be re-established, if it be so, by the collapse of 

 alternative explanations which are more easily attacked by observation 

 And experiment. 



If so, it will present a parallel to the Theory of Evolution itself, a theory 

 universally accepted, not because it can be proved by logically coherent 

 evidence to be true, but because the only alternative, special creation, is 

 clearly incredible. 



The alternative explanations which are put forward of the existence 

 of the differences which separate species from species or one geographical 

 race from another are in essence three : either the difference is regarded as 

 adaptive and its initiation and perfectioning are attributed to a reaction 

 of the animal which alters its structure in a favourable manner followed by 

 an inheritance of the character so acquired, or, secondly, it is regarded as non- 

 adaptive, or only accidentally of value, and held to have arisen by a change 

 induced in the course of an individual development by the direct effect of 

 some one or more environmental features, such change not necessarily 

 being heritable in all cases. The third explanation is that the difference 

 between one form and the other has arisen casually, isolation having 

 enforced an inbreeding which led to the distribution of genes in different 

 proportions in the two stocks. 



The first alternative explanation suft'ers from the defect that the 

 ■characters in question have not in general been shown to be adaptive, and 

 that an inheritance of an acquired character of the kind required has not 

 been shown to occur. 



The second explanation, the direct influence of the environment, has 

 the immense advantage that it is open to investigation by experimental 

 methods, and suggests many attractive lines of work. 



Here again experiments have been few. The most successful is that 



on the induction of melanism in moths by Heslop Harrison and Garrett. 



By feeding caterpillars on food impregnated by salts of manganese or 



lead, these authors, in three independent series of experiments, obtained 



melanic individuals of a character which did not occur in the much larger 



