December 12, 1884.] 



SCIENCE. 



533 



causes an excess of female births, while an unfavor- 

 able environment causes an excess of male births. 



Among mankind the conditions of life are so much 

 under control, that it is difficult to say just what con- 

 stitutes a favorable environment; but I think we 

 may safely conclude that a high birth-rate indicates 

 that the conditions of life are favorable, and that a 

 decrease in the birth-rate indicates decreased pros- 

 perity, and that human races which are disappearing 

 are so doing because surrounding conditions are no 

 longer favorable. 



Diising gives many facts to show, that, as the birth- 

 rate increases, the number of boy-births to each 100 

 girl-births decreases, and vice versa. At the Cape of 

 Good Hope the Boers are very prolific : six or seven 

 is a small family, and from twelve to twenty children 

 are not unusual, and 100 girls are born to every 97.2 

 boys. The Hottentots, on the other hand, are very 

 infertile: many of the women are barren, and they 

 seldom have more than three children, and 103.9 boys 

 are born to each 100 girls. 



The birth-rate is higher in towns than it is in the 

 country, and the ratio of boys is greater in the coun- 

 try than it is in the towns. In 1881 the average for 

 the whole of Prussia was'106.36 boys to each 100 girls ; 

 and in all the towns the boy-births were below this 

 average, and above the average in the country. Ploss 

 has shown that in Saxony the ratio of boy-births 

 rises and falls with the price of food. 



From nearly 10,000,000 births, Diising has compiled 

 a table to show the birth-rate, and the ratio between 

 the sexes, for each month in the year; and this table 

 shows that the ratio of boy-births is the highest 

 when the birth-rate is lowest. In March the birth- 

 rate was highest (942,488), and the ratio of boy-births 

 was lowest (105.92 boys to each 100 girls); while in 

 June the birth-rate was lowest (812,469), and the ratio 

 of boys highest (106.77). 



Among the lower animals, it is difficult to obtain 

 statistics : but Dusing states that domesticated animals 

 are more prolific than their wild allies, and that there 

 is a greater number of female births; that, when 

 animals are taken from a warm to a cold climate, the 

 ratio of male births increases ; and that leather-dealers 

 state that they obtain most female skins from fertile 

 regions with rich pastures, and most male skins from 

 more barren countries. 



The power of parthenogenetic reproduction seems, 

 in many cases, to have been acquired in order to 

 permit an unusually great and rapid increase in the 

 birth-rate, when the conditions of life are unusually 

 favorable ; and in these cases the parthenogenetic eggs 

 give birth to females almost exclusively. Among the 

 parthenogenetic Cladocera, both males and females 

 are found in the fall and in the early spring; but dur- 

 ing the warm months only females are found, and 

 they multiply so rapidly, that, according to Kamdohr, 

 a female Daphnia can in sixty days produce 1,291,- 

 370,075 parthenogenetic female descendants. As the 

 supply of food fails in the fall, males make their ap- 

 pearance; and Kurz has shown that any unfavorable 

 change causes the production of males. He says 

 that males appear when food fails, when the water 



dries up, when it becomes too dense, when it acquires 

 an unfavorable temperature, or, in general, when there 

 is a decrease in prosperity. From these and many 

 other facts recorded by Dusing, I think we may safely 

 conclude, that among animals and plants, as well as 

 in mankind, an unfavorable environment causes an 

 excess of male births, and a favorable environment 

 an excess of female births. 



Now, why should this be so? If the welfare of the 

 species can be secured, under a favorable environ- 

 ment, by females alone, why are males needed when 

 the environment becomes unfavorable ? I believe 

 that we have, in the facts recorded by Dusing, an illus- 

 tration of one of the most important and far-reaching 

 of all the adaptations of nature, — an adjustment 

 which tends to cause variation when it is needed, and 

 to keep things as they are, so long as no change is 

 demanded. As the conditions of life become unfavor- 

 able, variation becomes desirable in order to restore 

 the adjustment between the organism and its envi- 

 ronment ; and this is secured by an increase in the 

 ratio of male births. 



That this is the^true explanation of the phenomena, 

 is shown, I think, by the contrast between domes- 

 ticated animals and captive animals. The fact that 

 an animal has become domestic shows that it finds in 

 captivity a favorable environment; and Dusing says 

 that domestic animals are exceptionally fertile, and 

 that they produce an excess of females. Animals 

 which are kept as captives in menageries and gardens, 

 have, as a rule, no fitness for domestication; and 

 Geoffroy St. Hilaire says that individuals born in 

 menageries are usually male, while skins sent to 

 museums are usually female; and that the attempt 

 to domesticate a wild animal increases the number of 

 male births. Dusing states that captive birds of prey, 

 and carnivorous mammals, are very infertile, and that 

 the young are nearly always males. 



The wild human races of Oceanica and America are 

 much like captive animals, as they have been sud- 

 denly thrown into contact with a civilization which 

 has been in Europe the slow growth of thousands of 

 years. Food and climate have not changed, but a 

 new element has been introduced into their environ- 

 ment. The New-Zealanders are very infertile, and 

 nearly all the children are boys ; and the census of 

 1872 for the Hawaiian Islands gave a ratio of 125 

 male births to each 100 female births. 



I believe we may see, in these instances, the last 

 struggle of nature to save the race from extermina- 

 tion by the production of a favorable variation. It 

 is proper, however, to point out that Dusing himself 

 gives a different explanation of the excess of male 

 births under unfavorable conditions of life, although 

 I believe that examination will show that his expla- 

 nation is inadequate. 



He says that the excess of male births is for the 

 purpose of preventing close inter-breeding. He shows 

 that inter-breeding causes sterility, small size, and 

 lack of general vigor and vitality; and he also shows 

 that these effects are most marked when the other 

 conditions of life are least favorable, and that no evil 

 effects follow inter-breeding when food is abundant,. 



