THE DETERMINATION OF SEX. 49 



caution in such matters. A water-melon was grown in a heated 

 glass-house, where the temperature sometimes rose on warm 

 days to iio° Fahr. "The plant grew with equal health and 

 luxuriance, and afforded a most abundant blossom ; but all its 

 flowers were male. This result did not in any degree surprise 

 me, for I had many years previously succeeded, by long con- 

 tinued very low temperature, in making cucumber plants 

 produce female flowers only ; and I entertain but little doubt 

 that the same fruit stalks might be made, in this and the 

 preceding species, to support either male or female flowers in 

 obedience to external causes." 



This experiment was obviously more sanguine than satis- 

 factory. Heyer justly points out that of the water-melon only 

 a single plant was taken. Furthermore, he says, the water- 

 melon in nature usually bears only female flowers on the apices 

 of the older twigs, and may bear only a minimum number of 

 these. Knight's observations on cucumbers are also open to 

 serious objections, and were too scanty to prove anything. 



Meehan finds that the male plants of hazel grow more 

 actively in heat than the female ; and Ascherson has made the 

 interesting observation, that the water-soldier {Sfrafiotes aloides) 

 bears only female flowers north of 52° lat., and from 50° south- 

 wards only male ones. 



In the human species, Diising and others have noted that 

 more males are born during the colder months; and Schlechter 

 has reached the same results from observations upon horses. 

 The temperature of the time, not of birth but of sex determina- 

 tion, must of course be noted ; nor must it be forgotten that 

 temperature may have its influence indirectly through the 

 nutritive functions. 



§ 3. Summary of Factors. — -If we now sum up the case, it 

 must first be recognised that a number of factors co-operate in 

 the determination of sex; but that the most important of these, 

 with increasing penetration of analysis, may be more and more 

 resolved into plus or minus nutrition, operating upon parent, 

 sex elements, embryo, and in some cases larvEe. 



{a) Starting with the parent organisms themselves, we find 

 this general conclusion most probable, — that adverse circum- 

 stances, especially of nutrition, but also including age and the 

 like, tend to the production of males, the reverse conditions 

 favouring females. 



{b) As to the reproductive elements, a highly nourished 



D 



