NURTURAL INFLUENCES 499 



diverse seems at first sight to tell against the idea of an internal 

 automatic production of two kinds of gametes " against the 

 existence of an intrinsic and uniform mechanism of sex-produc- 

 tion and against the specific assumption that sex is transmitted 

 as a Mendelian character." But Prof. E. B. Wilson suggests 

 that this difficulty may be overcome by supposing that there 

 is a disproportion 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 be a certain proportion of impotent spermatozoa, as is 

 well known to be true of the pollen-grains of some flowering 

 plants, like Mirabilis. 



9. Fifth Theory : Thai environmental and functional in- 

 fluences, operating through the parent's body, may alter the 

 proportion of effective female-producing and male-producing 

 germ-cells 



This, like the first theory, admits the importance of nurture 

 (in the wide sense), but supposes it to be influential at an early 

 stage in determining the proportion of effective female-producing 

 and male-producing germ-cells. Supposing that the original 

 germ-cells are, as Mendelian theory would lead us to expect, 

 divided into two camps, male-producing and female-producing, 

 we can readily conceive that nurtural conditions may some- 

 times influence the relative rate of increase or the percentage of 

 survival in the two groups. Or supposing that the immature 

 germ-cells are constitutionally indifferent, as likely to develop 

 into males as into females, we can readily conceive that nurtural 

 conditions, such as a change in the nutritipn of the parent, may 

 sometimes decide their destiny. 



It seems fairly clear that there are many cases where this 

 theory of nurtural determination will not apply at all, e.g. when 

 numerous young are born at once and show an approximately 



