SKX 



SEXTANT 



viiliml for the production of these two kinds of 

 element* ; (</) the divergence In-twcen tlie sexes in 

 secondary as well as in essential characteristics ; 

 and (e) the evolution of the sexual in-iim-t- which 

 find their highest expression in the love ami court- 

 ship of many of the higher animals. A* die general 

 problem of sexual reproduction is discussed in tin- 

 article REPRODUCTION, and as the divergent modes 

 of sexual reproduction are discussed in special 

 articles, such as HEBMAPHRODITISM and GENERA- 

 TIONS (ALTERNATION OF), we shall contiiie our- 

 selves here to the problems immediately connected 

 with the evolution of sex. 



What is the meaning of the existence of two 

 distinct sexes? The prohlem may lie approached 

 in several different ways. We may, for instance, 

 inmiire whether this normal divergence of indi- 

 viduals into two types has any analogue among 

 other cases of divergent variation among organisms. 

 To some the analogy seems not far to seek. For 

 throughout organic nature a great contrast may be 

 read, between plant and animal, Gregarine and 

 Infiisorian, coral and jellyfish, Uirnacle and shrimp, 

 coccus-insect and fly, slug and Pteropod, tortoise 

 and lizard, reptile and bird, and even more clearly 

 between related genera or even related species ; 

 a contrast between predominant passivity and 

 activity, between a storing and constructive phy- 

 siological habit and one which is prone to expen- 

 diture anil disruption ; a contrast expressible in 

 technical language as an antithesis between a 

 relative anabolic and n relative kataliolic pre- 

 ponderance in the protoplasmic life of the creature. 

 Now it may be that the difference between the 

 sexes, between the relatively passive female and 

 the more energetic male, is an expression of what 

 is really a fundamental alternative in variation. 



Or we may take a survey of the contrasts be- 

 tween the sexes, and endeavour, after making 

 allowance for the special conditions of each case, 

 to reach some average truth. Then we find that 

 females tend to lie larger, more sluggish, less 

 brightly coloured and decorative, that they often 

 have a longer life and sometimes a higher tempera- 

 ture ; and that males, on the other hand, tend to 

 be smaller, more energetic, more brightly coloured 

 and decorative, and so on. Now, these character- 

 istics, when physiologically analysed, tend to 

 oorrnlmrate the theory that the females and males 

 are individuals of relatively anabolic and relatively 

 katabolic constitutions. 



Again, we may approach the problem by inquir- 

 ing into the physiological conditions which tend in 

 each birth to determine whether the individual will 

 lie a male or a female. In many cases e.g. in 

 amphibians, birds, and mammals the young life 

 is for some time usually of very short duration - 

 potentially bisexual : it is believed that surround- 

 ing conditions, especially of nutrition, acting upon 

 wnne constitutional predisposition, determine the 

 predominance of either maleness or femaleness. It 

 is true that we have hut incomplete knowledge of 

 this matter, but there are many facts which suggest 

 that influences of food, temperature, Vc., wliicli 

 favour analmlism in the parents, embryos, and 

 young, tend to result in female offspring, while 

 iipjMiMte factors tend to produce males. Thus by 

 abundant and rich nutrition .lung raised the per- 

 centage of females in a brood of tadpoles from 90 to 

 92, and starved caterpillars tend to liecome male 

 butterflies or moths. Or again, we may lix our atten- 

 tion on the characteristic products of the sexes on 

 the ova and spermatozoa. Here the difference be- 

 tween female and male has its fundamental and 

 most concentrated expression. For the ova are 

 relatively large cells, very passive, and usually rich 

 in reserve-products, while the spermatozoa are very 

 minute cells, usually very active, rarely with any 



reserve- prod net 8. In short, the ova (like the 

 females) are relatively .-timholii-, and the sperma- 

 tozoa (like the males) are relatively katabollc. 



We should also inquire into the Ix-ginning- i 

 sexual differentiation aiming the simpler forms of 

 life. Among the unicellular I'roto/oa. in which 

 sexual reproduction in the strict sense cannot 

 occur, there are already the analogues of males and 

 females, as we see Im instance in Voriicella, when 

 a small free-swimming individual unites with a 

 stalked cell of normal sixe. Very gradual among 

 the simple plants also ale the steps of sexual <lr 

 entiation. lint of especial importance is the genus 

 Volvox a colony of loosely united cells in many 

 ways related to the l-'lagellate Infusorians. In this 

 globular colony division of labour is at a minimum^ 

 and individuals often occur which are quite asexual, 

 forming daughter-colonies, without specialised re- 

 productive cells. Others have, among the other 

 units of the colony, special reproductive cells, which 

 are, however, pnrthcnogenetic, able of themselves 

 to form new colonies. Others have special repro- 

 ductive cells ova ami s|>einiato/.oa formed within 

 the same colony, but usually maturing at different 

 times. Then there are colonies- in which only ova 

 are formed, and others in which only uperMMMM 

 are formed, cross fertilisation taking place as usual. 

 And besides these various sexual conditions other 

 combinations often occur in the same species of 

 Volvox. iii which we can indeed read, and with 

 increasing physiological knowledge will more fully 

 understand, almost the whole story of the evolution 

 of sex. 



Finally, it must be observed that the origin p\ 

 two kinds of reproductive cells which combine in 

 fertilisation, ana the specialisation of two types of 

 Individual as the bearers of ova anil -pel m.-ito/.oa 

 respectively, must have lieen of advantage in the 

 I general evolution of organisms. For. as almost 

 every life begins in the mingling of two distinct 

 reproductive units produced by two distinct types 

 of individual, there is here one of the most im- 

 portant sources of variation and one of the most 

 important means of securing the average stability 

 of the species. 



But we must also recognise the psychical ex- 

 pression of sex the love of mates. This also 

 has its history. Among crustaceans and insects 

 first, in fishes and amphibians, in reptiles too, but 

 most markedly among birds and mammals, the 

 males are attracted to the females, and enter into 

 relations of helpfulness with them. The relations 

 and attractions may be crude enough to liegin 

 \\ith, but to mere physical fondness are added 

 subtler attractions of sight and hearing, and these 

 are sublimed in birds and mammals to what we 

 call love. This love of mates broadens out; it 

 laps the family in its folds ; it diffuses itself as 

 a saturating influence through the societies of 

 animals and of men. 



See REPRODUCTION, SEXUAL SELECTION, and The 

 Krolution of Stx, by Prof. Geddca and the present writer 

 ( 1889), with n bibliography of the subject. 



Sexagesimals. Bee SCALES OF NOTATION. 



Sexagesima Sunday (Lat. sexagesima, 

 'sixtieth'). See QUINQUA0ESIHA. 



Sextant, an instrument for measuring the 

 angular distance of object* by means of reflection. 

 The principle of ite construction depends ujion the 

 theorem that, if a ray of light suffer double reftec- 

 tion, the angle brttoeen the oriiinml m;/ ana itt 

 - 



after the srrtiiift rrflrctuni is i/nnhle of the 

 angle made by the rrflrrting mirfni'm. Thus, let A 

 and I! i fig. 1) In; two mirrors |-rpendicular to the 

 same plane, and inclined to each other, and let SA 

 be a ray of light, which falling upon A is reflected 

 on B, and re-reflected in the direction BC, then 



