登陆注册
35025500000045

第45章

After the foregoing discussion, which ought to have been much amplified, we may, I think, assume that the modified descendants of any one species will succeed by so much the better as they become more diversified in structure, and are thus enabled to encroach on places occupied by other beings. Now let us see how this principle of great benefit being derived from divergence of character, combined with the principles of natural selection and of extinction, will tend to act.

The accompanying diagram will aid us in understanding this rather perplexing subject. Let A to L represent the species of a genus large in its own country;these species are supposed to resemble each other in unequal degrees, as is so generally the case in nature, and as is represented in the diagram by the letters standing at unequal distances. I have said a large genus, because we have seen in the second chapter, that on an average more of the species of large genera vary than of small genera; and the varying species of the large genera present a greater number of varieties. We have, also, seen that the species, which are the commonest and the most widely-diffused, vary more than rare species with restricted ranges. Let (A) be a common, widely-diffused, and varying species, belonging to a genus large in its own country. The little fan of diverging dotted lines of unequal lengths proceeding from (A), may represent its varying offspring. The variations are supposed to be extremely slight, but of the most diversified nature;they are not supposed all to appear simultaneously, but often after long intervals of time; nor are they all supposed to endure for equal periods.

Only those variations which are in some way profitable will be preserved or naturally selected. And here the importance of the principle of benefit being derived from divergence of character comes in; for this will generally lead to the most different or divergent variations (represented by the outer dotted lines) being preserved and accumulated by natural selection.

When a dotted line reaches one of the horizontal lines, and is there marked by a small numbered letter, a sufficient amount of variation is supposed to have been accumulated to have formed a fairly well-marked variety, such as would be thought worthy of record in a systematic work.

The intervals between the horizontal lines in the diagram, may represent each a thousand generations; but it would have been better if each had represented ten thousand generations. After a thousand generations, species (A) is supposed to have produced two fairly well-marked varieties, namely a 1 and m 1. These two varieties will generally continue to be exposed to the same conditions which made their parents variable, and the tendency to variability is in itself hereditary, consequently they will tend to vary, and generally to vary in nearly the same manner as their parents varied. Moreover, these two varieties, being only slightly modified forms, will tend to inherit those advantages which made their common parent (A) more numerous than most of the other inhabitants of the same country;they will likewise partake of those more general advantages which made the genus to which the parent-species belonged, a large genus in its own country. And these circumstances we know to be favourable to the production of new varieties.

If, then, these two varieties be variable, the most divergent of their variations will generally be preserved during the next thousand generations.

And after this interval, variety a 1 is supposed in the diagram to have produced variety a 2, which will, owing to the principle of divergence, differ more from (A) than did variety a 1. Variety m 1is supposed to have produced two varieties, namely m 2 and s 2, differing from each other, and more considerably from their common parent (A). We may continue the process by similar steps for any length of time;some of the varieties, after each thousand generations, producing only a single variety, but in a more and more modified condition, some producing two or three varieties, and some failing to produce any. Thus the varieties or modified descendants, proceeding from the common parent (A), will generally go on increasing in number and diverging in character. In the diagram the process is represented up to the ten-thousandth generation, and under a condensed and simplified form up to the fourteen-thousandth generation.

But I must here remark that I do not suppose that the process ever goes on so regularly as is represented in the diagram, though in itself made somewhat irregular. I am far from thinking that the most divergent varieties will invariably prevail and multiply: a medium form may often long endure, and may or may not produce more than one modified descendant; for natural selection will always act according to the nature of the places which are either unoccupied or not perfectly occupied by other beings; and this will depend on infinitely complex relations. But as a general rule, the more diversified in structure the descendants from any one species can be rendered, the more places they will be enabled to seize on, and the more their modified progeny will be increased. In our diagram the line of succession is broken at regular intervals by small numbered letters marking the successive forms which have become sufficiently distinct to be recorded as varieties. But these breaks are imaginary, and might have been inserted anywhere, after intervals long enough to have allowed the accumulation of a considerable amount of divergent variation.

同类推荐
热门推荐
  • 美人玦

    美人玦

    她是水神幺女,神后是她姑,三界最美的是她娘,天生的富贵命,美人劫。三千多岁骗了一只呆鸟,不想乃是九天的皇子鹞;七千多岁咬了一个毛娃子,不想却是魔界少主陵。好死不死她偏看上了老她一万多岁的皇叔,自此应了那红颜祸水多薄命,坠了椿萱失了仙骨,白白受了千年的轮回,十数世的情劫。一滴真情泪,她飞升上神重返九天。神魔两界早是又沧海又桑田,当年的情根深种却愣是没变,这一个火海一个冰山,唔,夹缝求生,她如何吃得消?!
  • 铸天道之风雨欲来

    铸天道之风雨欲来

    天道之子已经过了六世,各位天道守护都已经对此感到了平常,马上就要开始了下一世的轮回,但规则也越来越不稳定;为了稳定规则,四位规则至高神开始了闭关,把控制‘暗’的事情交由其他的至高神来主持,但他们失算了!这些至高神由于实力有限,最终还是没有完全的控制住,为天道之子的再一次轮回带来了未知·····
  • 主宰四界

    主宰四界

    这个世界终有你意想不到的事情,可能在你认为真实的世界上,在其他人眼里却是虚伪的。一个少年因为拥有最强血脉而被神族放入幻想世界修炼,从试炼之地出来后的他有会发生什么,一切都是未知数。
  • 英语PARTY——交际口语排练室

    英语PARTY——交际口语排练室

    本套书籍带你领略英语世界风景,感悟英语学习氛围,有助于英语学习。
  • 天行

    天行

    号称“北辰骑神”的天才玩家以自创的“牧马冲锋流”战术击败了国服第一弓手北冥雪,被誉为天纵战榜第一骑士的他,却受到小人排挤,最终离开了效力已久的银狐俱乐部。是沉沦,还是再次崛起?恰逢其时,月恒集团第四款游戏“天行”正式上线,虚拟世界再起风云!
  • 万界学院之先知是我

    万界学院之先知是我

    一封神秘的邀请函,一个能够入侵万界的高校,当江寒得知这些位面都是地球的各种小说影视时。什么?赛亚人是无品体质?火影中木叶最差都被围攻了?江寒顿时知道,自己发达了。抱歉啊,这本书就这样了,我再开一本吧。那就好高考完,心情不太好。又被父母骂了,所以才放弃。现在想续上也没思路了。最近有了其他思路,想着再开一本书,如果合你的口味,可以来看看。再次道歉!
  • 符道

    符道

    大道五十,天衍四九,人遁其一。山村中走出的少年,该如何把握那一丝天机,借符文之力,万道入体,窥破那万载无人踏入的未知之境。
  • 末世之淡定生活

    末世之淡定生活

    在这样一个风雨交加、路上行人寥寥的夜晚,简佳莫名进入了一个用精神力开创出来的空间。开创这个空间的人是一位修为很高的从异世而来的人,这个空间本来是给他在地球上找到的道侣用的,现在因为世界末日就要来了,他们决定离开地球前往另一个空间。于是这个空间就被简佳接收……情节虚构,切勿模仿
  • 重生之复仇女王再度归来

    重生之复仇女王再度归来

    她们因复仇而走在一起,一起经历了很多的困难。终于再多年以后变得强大了,她们回来了,回来复仇了。复仇路程的艰难,她们都一步步挺过来了,但是复仇之后遇到的爱情,她们又经历了许多挫折,因误会而分开,又因大意而被暗杀,有再次重生了,回来后的她们是否能不再错过爱情,是否能解除误会?
  • 女尊之双夫临门

    女尊之双夫临门

    21世纪的她,劫后余生却穿越成了女尊世界的将军!手握兵权,声名显赫,却偏偏连自己的男人都护不住!“我不要我的男人在别的女人身下辗转承欢!”她出兵平乱,计除奸佞,血雨腥风的乱世,怒发冲冠只为蓝颜!然而,卸甲归田,双夫临门,一正太一腹黑,争宠吃醋,她还能否HOLD住!