登陆注册
4707200000045

第45章

Some authors have tried to show that the theory of mutation is opposed to Darwin's views. But this is erroneous. On the contrary, it is in fullest harmony with the great principle laid down by Darwin. In order to be acted upon by that complex of environmental forces, which Darwin has called natural selection, the changes must obviously first be there. The manner in which they are produced is of secondary importance and has hardly any bearing on the theory of descent with modification. ("Life and Letters"II. 125.)

A critical survey of all the facts of variability of plants in nature as well as under cultivation has led me to the conviction, that Darwin was right in stating that those rare beneficial variations, which from time to time happen to arise,--the now so-called mutations--are the real source of progress in the whole realm of the organic world.

II. EXTERNAL AND INTERNAL CAUSES OF VARIABILITY.

All phenomena of animal and plant life are governed by two sets of causes;one of these is external, the other internal. As a rule the internal causes determine the nature of a phenomenon--what an organism can do and what it cannot do. The external causes, on the other hand, decide when a certain variation will occur, and to what extent its features may be developed.

As a very clear and wholly typical instance I cite the cocks-combs (Celosia). This race is distinguished from allied forms by its faculty of producing the well-known broad and much twisted combs. Every single individual possesses this power, but all individuals do not exhibit it in its most complete form. In some cases this faculty may not be exhibited at the top of the main stem, although developed in lateral branches: in others it begins too late for full development. Much depends upon nourishment and cultivation, but almost always the horticulturist has to single out the best individuals and to reject those which do not come up to the standard.

The internal causes are of a historical nature. The external ones may be defined as nourishment and environment. In some cases nutrition is the main factor, as, for instance, in fluctuating variability, but in natural selection environment usually plays the larger part.

The internal or historical causes are constant during the life-time of a species, using the term species in its most limited sense, as designating the so-called elementary species or the units out of which the ordinary species are built up. These historical causes are simply the specific characters, since in the origin of a species one or more of these must have been changed, thus producing the characters of the new type. These changes must, of course, also be due partly to internal and partly to external causes.

In contrast to these changes of the internal causes, the ordinary variability which is exhibited during the life-time of a species is called fluctuating variability. The name mutations or mutating variability is then given to the changes in the specific characters. It is desirable to consider these two main divisions of variability separately.

In the case of fluctuations the internal causes, as well as the external ones, are often apparent. The specific characters may be designated as the mean about which the observed forms vary. Almost every character may be developed to a greater or a less degree, but the variations of the single characters producing a small deviation from the mean are usually the commonest. The limits of these fluctuations may be called wide or narrow, according to the way we look at them, but in numerous cases the extreme on the favoured side hardly surpasses double the value of that on the other side. The degree of this development, for every individual and for every organ, is dependent mainly on nutrition. Better nourishment or an increased supply of food produces a higher development; only it is not always easy to determine which direction is the fuller and which is the poorer one. The differences among individuals grown from different seeds are described as examples of individual variability, but those which may be observed on the same plant, or on cuttings, bulbs or roots derived from one individual are referred to as cases of partial variability. Partial variability, therefore, determines the differences among the flowers, fruits, leaves or branches of one individual: in the main, it follows the same laws as individual variability, but the position of a branch on a plant also determines its strength, and the part it may take in the nourishment of the whole. Composite flowers and umbels therefore have, as a rule, fewer rays on weak branches than on the strong main ones. The number of carpels in the fruits of poppies becomes very small on the weak lateral branches, which are produced towards the autumn, as well as on crowded, and therefore on weakened individuals. Double flowers follow the same rule, and numerous other instances could easily be adduced.

Mutating variability occurs along three main lines. Either a character may disappear, or, as we now say, become latent; or a latent character may reappear, reproducing thereby a character which was once prominent in more or less remote ancestors. The third and most interesting case is that of the production of quite new characters which never existed in the ancestors. Upon this progressive mutability the main development of the animal and vegetable kingdom evidently depends. In contrast to this, the two other cases are called retrogressive and degressive mutability. In nature retrogressive mutability plays a large part; in agriculture and in horticulture it gives rise to numerous varieties, which have in the past been preserved, either on account of their usefulness or beauty, or simply as fancy-types. In fact the possession of numbers of varieties may be considered as the main character of domesticated animals and cultivated plants.

同类推荐
  • 金箓设醮仪

    金箓设醮仪

    本书为公版书,为不受著作权法限制的作家、艺术家及其它人士发布的作品,供广大读者阅读交流。汇聚授权电子版权。
  • JOHN BARLEYCORN

    JOHN BARLEYCORN

    本书为公版书,为不受著作权法限制的作家、艺术家及其它人士发布的作品,供广大读者阅读交流。汇聚授权电子版权。
  • 丹霞子淳禅师语录

    丹霞子淳禅师语录

    本书为公版书,为不受著作权法限制的作家、艺术家及其它人士发布的作品,供广大读者阅读交流。汇聚授权电子版权。
  • 德风禅师般若语录

    德风禅师般若语录

    本书为公版书,为不受著作权法限制的作家、艺术家及其它人士发布的作品,供广大读者阅读交流。汇聚授权电子版权。
  • 懊憹门

    懊憹门

    本书为公版书,为不受著作权法限制的作家、艺术家及其它人士发布的作品,供广大读者阅读交流。汇聚授权电子版权。
热门推荐
  • 重生后我把夫君休了

    重生后我把夫君休了

    大婚夜被人设计,第二天她便被新婚夫君休出门。半年后,她查清真相,带着一个粉雕玉琢的娃儿揪出了幕后黑手。原来当初陷害她的人竟然是……“娘亲,他说他是我爹爹。”宝宝指着俊美邪气的妖界之王满眼好奇,她冷冷一瞥,眼神陌然,“宝贝,我们走,不要跟陌生人说话。”
  • 穿越之多情王爷杀手妃

    穿越之多情王爷杀手妃

    她,有点天真!作为杀手的她仍天真的相信爱情,在完成任务的她,与爱的人坐在湖边上谈论以后的生活!紫星之钻,无价之宝!却没想到他只是利用她,他一枪夺走她的生命,令他没想到的是:她倒下湖中,祡星之钻却在她身上,无价之宝与她一同坠入湖中…她发誓:如果有来生不想再爱…当她以为自已死的时,却发觉还活着,一场圣旨婚姻把她与他邦在一起!但他却有爱的人,他对她冷漠,她无视,原本‘她’无才无艺现在的她多才多艺,迷倒众生…她和他能否在一起?她和他爱的人斗智斗勇,又能有怎样的结局?…无意中她成为古代杀手的堂主,暗杀堂!老者传她绝世武功,带暗杀堂走向顶盛…他融化她的心,她爱他,但他却不明白他对她是不是爱!她融化的心再次冰封!而且她怀了他的孩子……
  • 云外云岫禅师语录

    云外云岫禅师语录

    本书为公版书,为不受著作权法限制的作家、艺术家及其它人士发布的作品,供广大读者阅读交流。汇聚授权电子版权。
  • 豪门逆媳,逃婚三恋曲

    豪门逆媳,逃婚三恋曲

    T市第一新闻,夏家三位小姐和冷家三位少爷同时逃婚了!但这件事却被两个集团息事宁人。夏家三小姐突然回国,这件事给了大众一个深深的疑惑,不好的预感开始。她是隐藏在面具之下的狐狸,眼眸里除了算计再找不出别的。当懦弱的她遇上儒雅的他,他给尽她温柔却又为了家族把她推向地狱。同时,其他两姐妹的情感遭到破坏,她褪去懦弱的伪装,将自己的风华绝代展现于世。再见,她是四大总裁中唯一的女总裁,他也是权势滔天的商场大亨,针尖对麦芒。家族危机,死去的莫离却在眼前,当年事情的真相,阴谋正在筹划,一道道困难摆在她们眼前。两姐妹的爱情得到了救赎,但是夏雪涵却走了,再也不回来了。她去了一个叫天堂的地方,那有她,有他,就够了。
  • 我想在游戏世界里当第一

    我想在游戏世界里当第一

    我是孤傲的大侠,我为侠骨柔情。我是浪荡的徒子,我为俗粉折腰…
  • 罗近溪先生明道录

    罗近溪先生明道录

    本书为公版书,为不受著作权法限制的作家、艺术家及其它人士发布的作品,供广大读者阅读交流。汇聚授权电子版权。
  • 皓月与艳阳

    皓月与艳阳

    有没有一位故人是你想要重新联络的?卡洛琳·戴维斯·怀特并不是在寻找米基——现在的麦克·福斯特——那个她小时候的迷恋对象;她只是在逃避那个寻花问柳的丈夫,以求得安宁和时间来思考如何改变自己的人生。但麦克出现了,再一次,将她从不幸中拯救出来,前所未有的,他甚至比从前更英俊了。著名艺术家阳光,以为能逃回童年时夏天住过的小木屋,并躲在里面,但她错了。她的丈夫不愿放她离开。布莱德·怀特的家族里,从来没有人离过婚,一次都没有!而他现在也没有打算打破这个传统。卡罗琳能让他动摇吗?而麦克又会怎样呢?他会在她的生命里扮演怎样的角色?
  • 日本人与日本论

    日本人与日本论

    从日本的自然条件、历史渊源说起,通过分析近现代日本国家政治、经济等,指出了日本民族精神上的空虚与矛盾;对日本社会文化的方方面面做了分析。被学界视为研究日本的一本标杆性的著作,堪称中国的《菊与刀》。《日本人与日本论》 中国版的菊与刀,了解日本文化的必读之作。
  • 人间(中)

    人间(中)

    “《谁是我》中,我发现我不是我,我质问谁才是我?我历尽艰辛终于找到自我,我却痛苦逃避真实的我。兰陵王面具神秘依旧,又是谁躲在蓝衣社的背后?我携带密令,远赴美国,却遭致命阴谋,被判终身监禁。肖申克州立监狱,我已死为幽灵,又复活为英雄。人间的传奇正在继续……”
  • 三宝太监西洋记

    三宝太监西洋记

    本书为公版书,为不受著作权法限制的作家、艺术家及其它人士发布的作品,供广大读者阅读交流。汇聚授权电子版权。