I would like in a parenthetical section to expand and render rather more concrete this idea of the species as one divaricating flow of blood, by an appeal to its arithmetical aspect. I do not know if it has ever occurred to the reader to compute¹ the number of his living ancestors at some definite date, at, let us say, the year one of the Christian² era. Everyone has two parents and four grandparents, most people have eight great-grandparents, and if we ignore the possibility of intermarriage we shall go on to a fresh power of two with every generation, thus:—

Column 1: Number of generations.

Column 2: Number of ancestors.
3:  8
4:  16
5:  32
7:  128
10:  1,024
20:  126,976
30:  15,745,024

40: 1,956,282,976

I do not know whether the average age of the parent at the birth of a child under modern conditions can be determined³ from existing figures. There is, I should think, a strong presumption⁴ that it has been a rising age. There may have been a time in the past when most women were mothers in their early teens and bore most or all of their children before thirty, and when men had done the greater part of their procreation before thirty-five; this is still the case in many tropical climates, and I do not think I favour my case unduly⁵ by assuming that the average parent must be about, or even less than, five and twenty. This gives four generations to a century. At that rate and DISREGARDING INTERMARRIAGE OF RELATIONS the ancestors living a thousand years ago needed to account for a living person would be double the estimated population of the world. But it is obvious that if a person sprang from a marriage of first cousins, the eight ancestors of the third generation are cut down to six; if of cousins at the next stage, to fourteen in the fourth. And every time that a common pair of ancestors appears in any generation, the number of ancestors in that generation must be reduced by two from our original figures, or if it is only one common ancestor, by one, and as we go back that reduction will have to be doubled, quadrupled and so on. I daresay that by the time anyone gets to the 8916 names of his Elizabethan ancestors he will find quite a large number repeated over and over again in the list and that he is cut down to perhaps two or three thousand separate persons. But this does not effectually invalidate my assumption that if we go back only to the closing years of the Roman Republic, we go back to an age in which nearly every person living within the confines of what was then the Roman Empire who left living offspring must have been ancestral to every person living within that area to-day. No doubt they were so in very variable measure. There must be for everyone some few individuals in that period who have so to speak intermarried with themselves again and again and again down the genealogical series, and others who are represented by just one touch of their blood. The blood of the Jews, for example, has turned in upon itself again and again; but for all we know one Italian proselyte in the first year of the Christian era may have made by this time every Jew alive a descendant of some unrecorded

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1 compute
v./n.计算，估计
参考例句：
I compute my losses at 500 dollars.我估计我的损失有五百元。 The losses caused by the floods were beyond compute.洪水造成的损失难以估量。

2 Christian
adj.基督教徒的；n.基督教徒
参考例句：
They always addressed each other by their Christian name.他们总是以教名互相称呼。 His mother is a sincere Christian.他母亲是个虔诚的基督教徒。

3 determined
adj.坚定的；有决心的
参考例句：
I have determined on going to Tibet after graduation.我已决定毕业后去西藏。 He determined to view the rooms behind the office.他决定查看一下办公室后面的房间。

4 presumption
n.推测，可能性，冒昧，放肆，[法律]推定
参考例句：
Please pardon my presumption in writing to you.请原谅我很冒昧地写信给你。 I don't think that's a false presumption.我认为那并不是错误的推测。

5 unduly
adv.过度地，不适当地
参考例句：
He did not sound unduly worried at the prospect.他的口气听上去对前景并不十分担忧。 He argued that the law was unduly restrictive.他辩称法律的约束性有些过分了。

6 bastard
n.坏蛋，混蛋；私生子
参考例句：
He was never concerned about being born a bastard.他从不介意自己是私生子。 There was supposed to be no way to get at the bastard.据说没有办法买通那个混蛋。

7 pervade
v.弥漫，遍及，充满，渗透，漫延
参考例句：
Science and technology have come to pervade every aspect of our lives.科学和技术已经渗透到我们生活的每一个方面。 The smell of sawdust and glue pervaded the factory.工厂里弥漫着锯屑和胶水的气味。

8 segregated
分开的； 被隔离的
参考例句：
a culture in which women are segregated from men 妇女受到隔离歧视的文化 The doctor segregated the child sick with scarlet fever. 大夫把患猩红热的孩子隔离起来。

9 defer
vt.推迟，拖延；vi.(to)遵从，听从，服从
参考例句：
We wish to defer our decision until next week.我们希望推迟到下星期再作出决定。 We will defer to whatever the committee decides.我们遵从委员会作出的任何决定。

10 isolated
adj.与世隔绝的
参考例句：
His bad behaviour was just an isolated incident. 他的不良行为只是个别事件。 Patients with the disease should be isolated. 这种病的患者应予以隔离。

11 primitives
原始人（primitive的复数形式）
参考例句：
Almost all operators work only with primitives. 几乎所有运算符都只能操作“主类型”(Primitives)。 The anthropology of the future will not be concerned above all else with primitives. 未来的人类学不会以原始人为主要的研究对象。

12 edifying
adj.有教训意味的，教训性的，有益的v.开导，启发( edify的现在分词 )
参考例句：
Young students are advised to read edifying books to improve their mind. 建议青年学生们读一些陶冶性情的书籍，以提高自己的心智。 来自辞典例句 This edifying spectacle was the final event of the Governor's ball. 这个有启发性的表演便是省长的舞会的最后一个节目了。 来自辞典例句

13 motives
n.动机，目的( motive的名词复数 )
参考例句：
to impeach sb's motives 怀疑某人的动机 His motives are unclear. 他的用意不明。

14 mingle
vt.使混合，使相混；vi.混合起来；相交往
参考例句：
If we mingle with the crowd,we should not be noticed.如果我们混在人群中，就不会被注意到。 Oil will not mingle with water.油和水不相融。

15 hoarder
n.囤积者，贮藏者
参考例句：
Was I becoming an eccentric hoarder? 是我变成了一个古怪的收藏者吗？ 来自互联网

16 Founder
n.创始者，缔造者
参考例句：
He was extolled as the founder of their Florentine school.他被称颂为佛罗伦萨画派的鼻祖。 According to the old tradition,Romulus was the founder of Rome.按照古老的传说，罗穆卢斯是古罗马的建国者。

17 lapse
n.过失，流逝，失效，抛弃信仰，间隔；vi.堕落，停止，失效，流逝；vt.使失效
参考例句：
The incident was being seen as a serious security lapse.这一事故被看作是一次严重的安全疏忽。 I had a lapse of memory.我记错了。

18 degenerate
v.退步，堕落；adj.退步的，堕落的；n.堕落者
参考例句：
He didn't let riches and luxury make him degenerate.他不因财富和奢华而自甘堕落。 Will too much freedom make them degenerate?太多的自由会令他们堕落吗？

19 dependence
n.依靠，依赖；信任，信赖；隶属
参考例句：
Doctors keep trying to break her dependence of the drug.医生们尽力使她戒除毒瘾。 He was freed from financial dependence on his parents.他在经济上摆脱了对父母的依赖。

20 antagonists
对立[对抗] 者，对手，敌手( antagonist的名词复数 )； 对抗肌； 对抗药
参考例句：
The cavalier defeated all the antagonists. 那位骑士打败了所有的敌手。 The result was the entire reconstruction of the navies of both the antagonists. 双方的海军就从这场斗争里获得了根本的改造。

21 diluted
无力的，冲淡的
参考例句：
The paint can be diluted with water to make a lighter shade. 这颜料可用水稀释以使色度淡一些。 This pesticide is diluted with water and applied directly to the fields. 这种杀虫剂用水稀释后直接施用在田里。

22 overridden
越控( override的过去分词 )； （以权力）否决； 优先于； 比…更重要
参考例句：
The chairman's veto was overridden by the committee. 主席的否决被委员会推翻了。 Property '{0}' is not declarable, and cannot be overridden. 属性“{0}”是不可声明的，不能被重写。

23 entirely
ad.全部地，完整地；完全地，彻底地
参考例句：
The fire was entirely caused by their neglect of duty. 那场火灾完全是由于他们失职而引起的。 His life was entirely given up to the educational work. 他的一生统统献给了教育工作。

24 vividly
adv.清楚地，鲜明地，生动地
参考例句：
The speaker pictured the suffering of the poor vividly.演讲者很生动地描述了穷人的生活。 The characters in the book are vividly presented.这本书里的人物写得栩栩如生。

25 illuminating
a.富于启发性的，有助阐明的
参考例句：
We didn't find the examples he used particularly illuminating. 我们觉得他采用的那些例证启发性不是特别大。 I found his talk most illuminating. 我觉得他的话很有启发性。

26 dominant
adj.支配的，统治的；占优势的；显性的；n.主因，要素，主要的人(或物)；显性基因
参考例句：
The British were formerly dominant in India.英国人从前统治印度。 She was a dominant figure in the French film industry.她在法国电影界是个举足轻重的人物。

27 recessive
adj.退行的，逆行的，后退的，隐性的
参考例句：
Blue eyes are recessive and brown eyes are dominant.蓝眼睛是隐性的；而褐色眼睛是显性的。 Sickle-cell anaemia is passed on through a recessive gene.镰状细胞贫血通过隐性基因遗传给后代。

28 poetical
adj.似诗人的；诗一般的；韵文的；富有诗意的
参考例句：
This is a poetical picture of the landscape. 这是一幅富有诗意的风景画。 来自《简明英汉词典》 John is making a periphrastic study in a worn-out poetical fashion. 约翰正在对陈腐的诗风做迂回冗长的研究。 来自辞典例句

29 chaotic
adj.混沌的，一片混乱的，一团糟的
参考例句：
Things have been getting chaotic in the office recently.最近办公室的情况越来越乱了。 The traffic in the city was chaotic.这城市的交通糟透了。

30 correlation
n.相互关系，相关，关连
参考例句：
The second group of measurements had a high correlation with the first.第二组测量数据与第一组高度相关。 A high correlation exists in America between education and economic position.教育和经济地位在美国有极密切的关系。

31 immortal
adj.不朽的；永生的，不死的；神的
参考例句：
The wild cocoa tree is effectively immortal.野生可可树实际上是不会死的。 The heroes of the people are immortal!人民英雄永垂不朽！