SUMMARY OF CURRENT RESEARCHES. 559 



there is a markedly increased proportion of females. There are two 

 sharply-defined breeding seasons each year (in Cuba), and reference to 

 records of temperature, barometric pressure, humidity, etc., shows that 

 these bursts of reproductive activity always take place at times when 

 there is a marked change of climate — the one in autumn, shortly after 

 a sudden change from great heat to cooler weather ; the other in the 

 early months of the year, when the cool winter weather gives place to 

 spring. The greatest excess of females is produced at times of greatest 

 fertility, i.e., during the breeding season, when the metabolism of the 

 mother is most active. The proportion of females born in towns is 

 considerably higher than in country districts. 



"While whites show a more marked sensibility to the influences 

 which induce the production of females, coloured people are more 

 affected by the forces which stimulate the production of males, and 

 this condition is more marked among illegitimate than among legitimate 

 birth records. This fact shows that the race which normally produces 

 a considerable excess of males is most amenable to the forces which 

 induce the ripening of female ova, while the race which produces the 

 greatest proportion of females reacts more generously to the influences 

 which favour the production of male ova. In other words, there is 

 demonstrated the exercise of a force which limits the power to produce 

 an excess of either sex, a force which makes for some point near 

 equality of the sexes." This is the force of heredity. 



" Extraneous forces undoubtedly exist which effect a variation in 

 the sex ratio, but they are to some extent subordinate to laws of 

 heredity ; nevertheless, these former forces cannot be ignored : they are 

 certain to interfere to some extent with the performance of the laws of 

 heredity, and with all calculations regarding sex ratio which are based 

 solely upon those laws." 



To sum up the thesis : — " The variable metabolic activity of the 

 mother, acting upon the ovary, induces a struggle for existence between 

 the ovarian ova of different sexes, and affects the proportion of male 

 and female ova which ripen, and which are produced for fertilisation." 



" These same extraneous forces must affect the proportionate pro- 

 duction of individuals possessing various kinds of different characters 

 (quite other than sex) which are associated with metabolism, and, when 

 better understood, may have valuable bearing on the means for selection 

 of healthy ova and for preventing the maturation of ova bearing the 

 active germs of disease." 



Breeding Experiments with Rats.* — T. H. Morgan has bred the 

 black rat (Mus rattus) and the roof rat or Alexandrian rat, which has 

 the ticked or barred grey hair common to most wild rodents. It may be 

 looked on as the original form from which the black rat has been 

 derived. The hybrids were black. These inbred have, as yet, produced 

 only one litter, four black and one grey. It seems that the two colours 

 follow Mendel's law. 



" If the black colour in the black rat is produced by the loss of the 

 ticking factor, as Castle suggests for other black rodents, it is in- 



• Arner. Nat., xliii. (1909) pp.' 182-5.' 



