THE DETERMINATION OF SEX. 43 



sexual differences; but the fact of superior constitutional vegetative - 

 ness in the females is here so peculiarly obvious that it can hardly 

 fail to arouse a strong impression that more or less advantageously 

 nutritive conditions, whether of the embryo or of the seedling, are 

 sufficient to account for the differences of sex. 



II. Influence of Temperature. — In this connection not a few 

 writers have referred to an observation by Knight, which, from its 

 comparatively ancient date, perhaps deserves to be recorded in his 

 own words, if only to show the necessity of caution in such matters. 

 A watermelon was grown in a heated glasshouse, where the tem- 

 perature sometimes rose on warm days to no° 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 continued 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 satisfactory. 

 Heyer justly points out that of the watermelon only a single plant 

 was taken. Furthermore, he says, the watermelon 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 the hazel grow more actively 

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

 observation that the water-soldier (Stratiotes aloides) bears only female 

 flowers north of 5 2° lat., and from 50 southwards 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-determination, must of course be 

 noted; nor must it be forgotten that temperature may have its influence 

 indirectly through the nutritive functions. 



III. Summary of Factors. — If we now sum up the case, it 

 must first be recognized that a number of factors cooperate 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 larvae. 



(a) Starting with the parent organisms themselves, we find this 



