The President's Address. By J. Arthur Thomson. 155 



are more positive here, e.g., the broad abdomen. " If the parasite 

 dies and the host recovers, the ovary of the female may again become 

 functional ; but in the male under such circumstances eggs may be 

 produced in the testis. Geoffrey Smith concludes from these obser- 

 vations and from others on the Cirripedes, that the female is homo- 

 zygous in sex and the male heterozygous. There seems no a priori 

 reason," Mr. Doncaster continues, " why this should not be true in 

 the case of Crustacea and flowering plants, while the converse is the 

 case in moths and Vertebrates." 



30. The fact that the proportions of the sexes are sometimes 

 very variable (as Heape points out in regard to canaries) does not 

 of itself tell against the. view that the ova are determined at an 

 early stage to be male- producers or female-producers. There may 

 be a process of discriminate selection during the maturing of the 

 ova, and we know that in higher Vertebrates the possible ova do 

 not all come to maturity. 



That the proportions of the sexes in different types are very 

 diverse seems at first si"ht to tell against the idea of an internal 

 automatic production of two kinds of gametes — " against the exist- 

 ence of an intrinsic and uniform mechanism of sex -production and 

 against the specific assumption that sex is transmitted as a Mende- 

 lian character." But Professor E. B. Wilson suggests that this 

 difficulty may be overcome by supposing that there is a dispropor- 

 tion in the number of one kind of spermatozoa (like that which 

 reaches a climax in Aphids, Daphnids, etc., where only the female- 

 producing spermatozoa are left), or that there is a certain proportion 

 of impotent spermatozoa, as is well known to be true of the pollen- 

 grains of some flowering plants, like Mirabilis. 



E. — Can environmental and functional influences, opera- 

 ting THROUGH THE PARENT, OR, IN SHORT, THE PARENT'S 

 ACQUIRED PECULIARITIES, ALTER THE PROPORTION OF EFFEC- 

 TIVE FEMALE-PRODUCING AND MALE-PRODUCING GERM-CELLS ? 



31. Supposing that an immature ovum is as likely to develop 

 into a male as into a female, we can conceive it possible that a 

 change in the nutrition of the parent may decide its destiny. Or, 

 going further back, supposing the original germ-cells are already, as 

 Mendelian theory would lead us to expect, divided into two camps, 

 male-producing and female-producing, it may be that environmental 

 conditions can influence the relative rate of increase and percentage 

 of survival in these two camps. 



32. From human statistics some have tried to prove that abun- 

 dant food favours the production of female offspring, and vice versa ; 

 but others have concluded, also from statistics, that the parental 

 nutrition is of no moment, unless in bringing about a differential 

 death-rate. The fact that 30 p.c. of human twins are of different 



