PRESIDENTIAL ADDRESS SECTION D. 89' 



teristics. Parkes has now worked out the cause of the preponder- 

 ance of one sex over another by genealogical investigations. Race. 

 also appears to be a factor. He states that "among the Jews, no 

 matter in what part of the world they may happen to be resident, 

 there invariably occurs a much greater normal excess of male 

 births over female than is the case with Christians." Also, during 

 the recent war, the ratio of male to female births rose steadily and 

 persistently. It may be assumed that the war was in some obscure 

 way beneficial to the welfare of the Y gametes. Also, "fluctuation 

 in the number of male births per 1,000 female during the last 

 century follows almost exactly the rise and fall in the economic 

 price of food. ... It is impossible for sex to be altered by 

 nutrition after conception, so ... we conclude that higher 

 economic prices, and, consequently, more hardships, are capable of 

 affecting the gametic ratios of the heterozygous sex, and of alter- 

 ing it in favour of the gametes with the male potentialities." 



Parkes' family analyses supply direct evidence that in man 

 the male is heterozygous for sex, while the female is homozygous. 

 The female has no determining influence on the sex of the off- 

 spring, and the characteristic of begetting a preponderance of one 

 sex over the other is an attribute of the male. 



The Processes of Evolution. 



Evolution may be defined as the gradual differentiation of 

 organisms from common ancestral forms. It is the only reason- 

 able explanation of the diversity of fossil and living beings. In 

 enquiring into the mechanism of evolution we must consider the 

 reciprocal influence — or action and reaction — of agents external 

 to the organism on the one hand and of the living substance itself 

 on the other. The external factors together constitute the environ- 

 ment and the internal factors are the specific properties of the 

 organism. The environment is the more easily analysed. The 

 two principal hypotheses proposed to explain evolution were both 

 based on the efficacy of external factors, namely, the hypothesis 

 of Lamarck in 1809 and the hypothesis of Darwin in 1859. 

 Lamarck's hypothesis begins with the conception that the struc- 

 ture of organisms is in harmony with the conditions under which 

 they lived and that it is adapted to these conditions. The organism 

 is sTiaped by the environment. Usage develops the organs in the 

 individual and without usage they become atrophied. The modi- 

 fications thus acquired are transmitted to posterity, i.e., acquired 

 characters are inherited. Lamarckism looked to the very cause of 

 the change or variation among organisms by its method of explain- 

 ing adaptation. 



Darwin in his later life admitted the theoretical importance 

 of adaptation and the inheritance of acouired characteristics, but 

 placed them in a position of secondarv importance in the accom- 

 plishment of evolution. Darwin found a basis in the variability 

 of organisms which he accepted as an observed fact, without trying 

 to discover the cause of variations. The individuals which pos- 

 sessed advantageous variations under the conditions in which they 

 lived had more chance to survive and to reproduce themselves. 



