272 



The Journal of Heredity 



were alike. Hence it becomes clear 

 that if once it is admitted that sexual 

 reproduction is necessary or ad\'an- 

 tageous to the organism, the distinction 

 of the two sexes, male and female, 

 follows almost inevitably." 



DIFFERENCES BETWEEN SEXES 



Although this distinction between 

 male and female as sperm-]jroducer and 

 egg-producer is the fundamental one 

 for our present purpose, there arc in 

 general great differences of other kinds 

 between the two sexes. The female 

 in most species is relatively quiescent, 

 passive, and devotes her time to storing 

 up energy, for the benefit of her young; 

 the male is "vigorous, restless and 

 active, and characterized by the dissipa- 

 tion of energy rather than the storing 

 of it." These physiological differences 

 lead to the production of striking 

 external differences, seen in the more 

 conspicuous adornment of the males 

 of most species, and the relatively 

 inconspicuous appearance of most 

 females; they even extend to mental 

 characteristics of human beings, "since 

 woman is said to be more receptive and 

 conservative, while men show greater 

 originality and are more inclined to- 

 wards change." 



In the adult animal, then, there is 

 usually no difficulty in telling at a 

 glance to which sex an individual 

 belongs; "even in species in which the 

 sexes are externally similar an examina- 

 tion of the mature reproductive organs 

 will always reveal the sex with cer- 

 tainty." But in the embryo this is 

 generally not the case: at an early, or 

 sometimes even a late stage, it is im- 

 pcjssible for the observer to say whether 

 the individual is going to become a 

 male or female. This puts sharply 

 before us the question on which students 

 have always been divided: whether at 

 such an early stage the sex is already 

 determined, or whether it is still to be 

 dctermiped by some environmental con- 

 dition or conditions. In other words, 

 is the sex determined when the egg and 

 si.)erm cells unite, or is the embryo then 

 and for some time afterward indifferent 

 in character, and aV)le to become; either 

 male or female, according to conditions? 



The latter theory has alwa\-s l^een the 

 more ]jopular one, but the Mendelian 

 school of genetists, and many of those 

 who have studied the cell with the 

 microscope, during recent years, incline 

 to believe that the sex is fixed, once for 

 all, when the ovum and spermatozoon 

 come together. Each side has strong 

 evidence and, as Dr. Don caster jjoints 

 out, it seems very unlikely that both 

 can be right; he therefore attempts to 

 find some sort of a reconciliation be- 

 tween them. 



PARTHENOGENESIS 



First we must consider cases in which 

 the sex is determined before the egg is 

 laid. vSuch is the case among those 

 insects, for example, where the repro- 

 duction is parthenogenetic — where the 

 egg is never fertilized by a sperm. In 

 such cases, it can often be i^redicted 

 with certainty whether a given egg 

 will result in a male or female. To 

 quote a single instance, the plant-louse 

 Phylloxera, which has so ravaged the 

 vineyards of Europe, produces two kinds 

 of eggs — large and .small. Neither kind 

 is fertilized by a male, but the former 

 always produce females, the latter 

 males. Among the Rotifers, and else- 

 where, an egg which would have yielded 

 a male if it had developed without 

 fertilization, produces a female if it 

 unites with a spermatozoon. The case 

 of the honey-bee is well known: queens 

 and workers are produced from fertil- 

 ized, drones from unfertilized eggs. 

 "All the eggs are as far as can be 

 discovered originally alike; all undergo 

 a similar maturation process, and the 

 sex of the offsjjring seems to depend 

 entirely on whether the egg-nucleus 

 conjugates with a sperm-nucleus, or 

 devclojis unfertilized." In this ea.se, 

 external conditions cannot determine 

 sex. Among many similar cases, that 

 of identical twins may be cited, as 

 observed to good advantage in man and 

 the armadillo. When twnns, quadrup- 

 lets, etc., are ])roduced by the division 

 of a single egg, rather than by the 

 fertilization of several separate eggs, 

 the individuals arc always of the same 

 sex, indicating that the sex must 

 alreadv be fixed immcdiatclv after the 



