532 LIFE : OUTLINES OF GENERAL BIOLOGY 



{e) FIFTH THEORY.— That environmental and functional influ- 

 ences, 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 of 

 two kinds, male-producing and female-producing, we can conceive 

 that nurtural conditions may sometimes influence the relative rate 

 of increase or the percentage of survival in the two groups. Or 

 supposing that the immature germ-cells are constitutionally indif- 

 ferent, we can conceive that nurtural conditions, such as a change 

 in the nutrition of the parent, may sometimes decide their destiny. 



It seems fairly clear that there are many cases to which this 

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

 numerous young are born at once and show an approximately equal 

 distribution of the sexes. 



On the other hand, there are cases where a mother produces a long 

 succession of offspring all of one sex, or produces one son and a long 

 succession of daughters, and so on. Such cases suggest that the con- 

 stitution of the parent may be of some importance, and we know 

 that the constitution is modifiable by nutrition and the other 

 factors in nurture. When we pass from general considerations such 

 as the above and appeal to the facts, we find an interesting conflict 

 of evidence. 



From human statistics some have tried to prove that abundant 

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

 others have concluded, also from statistics, that the parental nutri- 

 tion is of no moment, unless in bringing about a differential death- 

 rate. The fact that 30 per cent, of human twins are of different 

 sexes seems enough to show that the dieting of the parent is not of 

 great importance. 



Careful experiments have been made, e.g. by Cuenot and Schultze, 

 on the possible influence of the nutrition of the mammalian parent 

 (e.g. mouse) on the sex of the offspring; but the results are all 

 against the reality of this supposed influence, in which, however, 

 some breeders strongly believe. Schultze extended his experiments 

 over three generations, but the high feeding of grandparents as 

 well as parents did not seem to have any influence on the proportion 

 of the sexes among the offspring. 



Against these results, however, we have to balance the very 

 important work of Heape, who has brought forward evidence for 

 mammals and birds that peculiarities in nutrition and in other 

 environmental influences may exert a selective influence on the germ- 

 cells, affecting the proportion of male-producing and female- 

 producing gametes. 



