登陆注册
5618700000002

第2章

So with Dogs in their habits and instincts.It is a physiological peculiarity which leads the Greyhound to chase its prey by sight,--that enables the Beagle to track it by the scent,--that impels the Terrier to its rat-hunting propensity,--and that leads the Retriever to its habit of retrieving.These habits and instincts are all the results of physiological differences and peculiarities, which have been developed from a common stock, at least there is every reason to believe so.But it is a most singular circumstance, that while you may run through almost the whole series of physiological processes, without finding a check to your argument, you come at last to a point where you do find a check, and that is in the reproductive processes.For there is a most singular circumstance in respect to natural species--at least about some of them--and it would be sufficient for the purposes of this argument if it were true of only one of them, but there is, in fact, a great number of such cases--and that is, that, similar as they may appear to be to mere races or breeds, they present a marked peculiarity in the reproductive process.If you breed from the male and female of the same race, you of course have offspring of the like kind, and if you make the offspring breed together, you obtain the same result, and if you breed from these again, you will still have the same kind of offspring;there is no check.But if you take members of two distinct species, however similar they may be to each other and make them breed together, you will find a check, with some modifications and exceptions, however, which I shall speak of presently.If you cross two such species with each other, then,--although you may get offspring in the case of the first cross, yet, if you attempt to breed from the products of that crossing, which are what are called HYBRIDS--that is, if you couple a male and a female hybrid--then the result is that in ninety-nine cases out of a hundred you will get no offspring at all; there will be no result whatsoever.

The reason of this is quite obvious in some cases; the male hybrids, although possessing all the external appearances and characteristics of perfect animals, are physiologically imperfect and deficient in the structural parts of the reproductive elements necessary to generation.

It is said to be invariably the case with the male mule, the cross between the Ass and the Mare; and hence it is, that, although crossing the Horse with the Ass is easy enough, and is constantly done, as far as I am aware, if you take two mules, a male and a female, and endeavour to breed from them, you get no offspring whatever; no generation will take place.This is what is called the sterility of the hybrids between two distinct species.

You see that this is a very extraordinary circumstance; one does not see why it should be.The common teleological explanation is, that it is to prevent the impurity of the blood resulting from the crossing of one species with another, but you see it does not in reality do anything of the kind.There is nothing in this fact that hybrids cannot breed with each other, to establish such a theory; there is nothing to prevent the Horse breeding with the Ass, or the Ass with the Horse.So that this explanation breaks down, as a great many explanations of this kind do, that are only founded on mere assumptions.

Thus you see that there is a great difference between "mongrels," which are crosses between distinct races, and "hybrids," which are crosses between distinct species.The mongrels are, so far as we know, fertile with one another.But between species, in many cases, you cannot succeed in obtaining even the first cross: at any rate it is quite certain that the hybrids are often absolutely infertile one with another.

Here is a feature, then, great or small as it may be, which distinguishes natural species of animals.Can we find any approximation to this in the different races known to be produced by selective breeding from a common stock? Up to the present time the answer to that question is absolutely a negative one.As far as we know at present, there is nothing approximating to this check.In crossing the breeds between the Fantail and the Pouter, the Carrier and the Tumbler, or any other variety or race you may name--so far as we know at present--there is no difficulty in breeding together the mongrels.Take the Carrier and the Fantail, for instance, and let them represent the Horse and the Ass in the case of distinct species; then you have, as the result of their breeding, the Carrier-Fantail mongrel,--we will say the male and female mongrel,--and, as far as we know, these two when crossed would not be less fertile than the original cross, or than Carrier with Carrier.Here, you see, is a physiological contrast between the races produced by selective modification and natural species.I shall inquire into the value of this fact, and of some modifying circumstances by and by; for the present I merely put it broadly before you.

But while considering this question of the limitations of species, a word must be said about what is called RECURRENCE--the tendency of races which have been developed by selective breeding from varieties to return to their primitive type.This is supposed by many to put an absolute limit to the extent of selective and all other variations.

同类推荐
  • 濒湖炮炙法

    濒湖炮炙法

    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。
  • 解酲语

    解酲语

    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。
  • 无根树词注解

    无根树词注解

    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。
  • 归庐谭往录

    归庐谭往录

    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。
  • 佛说身观经

    佛说身观经

    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
  • 傀儡皇帝:汉献帝

    傀儡皇帝:汉献帝

    汉献帝,名刘协,字伯和。汉灵帝第三子,汉少帝弟。少帝被废后,被董卓迎立为帝。董卓被王允和吕布刺死后,董卓部将李傕等攻入长安,再次挟持了献帝。后献帝逃出长安,落到了曹手中。献帝在位31年,终被曹丕所废,后病死,终年54岁,葬于禅陵。《中国文化知识读本:傀儡皇帝(汉献帝)》我们讲述的就是这位汉朝末代皇帝的悲哀一生。
  • 倾城妻好美味

    倾城妻好美味

    【曼珠沙华出品】【感谢神画设计提供封面】奈何桥边,彼岸花开。五百年的孤苦等待,换来的却是他一句“妖孽”矛盾的对立,他们能否相爱。当一切打破了世俗,再次付出了所有,却始终逃避不了。残了身,伤了情,一刀两断。最终,再回头,命运的纠葛,是否还可以重来?
  • 浮屠三生之黄粱一梦

    浮屠三生之黄粱一梦

    我本佳人,奈何卿伤。一场烟雨山河,终是和黄粱一梦。如若爱君是错,我便心亡,但愿下一世我不在做你的徒弟,师傅......
  • 流离的萤火爱情

    流离的萤火爱情

    抬头看到的就是他那双孤傲的眼睛,散发着无数的寒气,让人不寒而栗,那张脸简直无懈可击,与哥哥相比似乎更胜一筹,但是他满脸的高傲和不屑,瞬间拒人于千里之外。那个冰山男依旧惜字如金,没有表情,我开始有些怀疑,老哥是不是认错人啦?呼呼,不理他们啦,走咯“答应我一个要求!”说得这么爽快?是早有预谋吗?可是不应该,总不至于他是策划者吧“要求?行,但是你不可以说…”委屈啊,莫名其妙地要答应冰山男一个要求。“不管如何,你都要信我!”那是你对我的乞求吗?一次次的错过,一次次的误会,他们之间是否经得起时间的考验?可爱善良的韩雪柔能够等到幸福钟声响起吗?面对昔日的男友、今时的未婚夫,她该如何抉择?求收藏,求推荐,求订阅,嘻嘻,我会再接再厉的~~~推荐——http://m.pgsk.com/a/450433/《邪魅总裁:女人,乖乖躺着!》推荐新作温馨治愈系列:听说,爱情回来过。http://m.pgsk.com/a/702512/
  • 霸道豪门:女人,别来无恙!

    霸道豪门:女人,别来无恙!

    “顾苏苏,你完了!”怀孕两个月,男友和闺蜜背叛将她逼上绝路。再醒来,她成了顶着无上光环的首席特工,身负重任混迹娱乐圈,却不想……“顾苏苏,你是我的女人!”某夜,男人强取豪夺之后,霸道地宣告着她的所有权,顾苏苏冷哼,“沈少,就你这连膜都破不了的长度?”可是,谁能告诉她,为什么当她任务失败的时候,这男人却搂着她的肩膀,语气暧昧,“她从此以后,你的任务就是伺候我!”
  • 天下第一儒术:圣人说

    天下第一儒术:圣人说

    儒家的处世之术早已成为世人可循的经验,千百年来,《论语》《孟子》更成为世人的必读经典。本书概括了儒学中众多精华的内容,分为“人而无信,不知其可”“仁者不忧,智者不惑”“贫而乐,富而好礼”“德不孤,必有邻”等章节。这些穿越时空的真知灼见,对我们的人生规划和为人处世都有着极其实用的指导意义。若能认真阅读,仔细参悟,当可为自己的修身立命找到一个可靠的解决途径。
  • 青荧

    青荧

    新文《人精要修仙》求支持哦! (这不是一本正经的古代重生文,有诅咒,有死神,有平行世界,但是无男主) 失去所有,坠入深渊从前所仰仗、拥有的,顷刻间化作齑粉……危机四伏,恶狼环伺且看祝荧于尘埃中重新崛起,天道不公,她就去争,去抢…….命都可以挣,还有什么赢不得?祝荧荧荧,光也——她,就是一道光;漆黑的人心,撕碎你,吃了你,怕谁?
  • 鬼帝宠妻

    鬼帝宠妻

    【一对一独宠】青素,北大经济学研究生,当界大学生散打冠军,路见不平拔刀相助,挂了……童颜,北宁王府嫡出小姐,刚入宫门封三品昭仪,不幸皇帝纵欲过度,殁了……青素醒来发现似乎死错了地方,她变成了被逼殉葬的亡妃‘童颜’。于是,第一次见面是这样的。在刚驾崩的皇帝陵墓中……他是已故先皇,活着的。她是殉葬亡妃,穿越的。她淡定地看着‘先帝’一双纤长玉手正在解开她的衣裳。嗯?这是要奸尸的情况?“先帝,我还是活的。”她忍着想一拳打扁那张妖孽般脸的冲动,冷冷道。‘先帝’一脸正经,“哦……这儿风光无限好,幽静而无人打扰,朕想起还没与爱妃圆房呢。”缓缓俯下身子,俊美无双的脸近在咫尺,“此处可好?朕的殉葬王妃。”“……”很好很好,的确无人,都是死鬼!为了活命她与伪鬼帝达成合作协议……古语云,有钱能使鬼推磨。斗斗王爷,翻翻皇陵,收收古玩,卖卖鬼帝,放放高利贷,就这样随便干点啥,省得无聊。待她傲立万里河山之巅,倾国倾城伪鬼帝摇着羽扇悠悠然,“老婆,可满意夫君一路保驾护航,助你纵横天下,颠覆皇权,挣得你这一生潋滟风华,璀璨无悔?”【魂魂PS:有夫如君,夫复何求】
  • 空岛纪元

    空岛纪元

    世界分崩离析,我们曾经熟悉的世界已经不复存在,人类,这个历经千年衍生下来的物种,将在这个空岛纪元中,继续存活
  • 剑仙的凡人生活

    剑仙的凡人生活

    有时候就是这么无聊,呢么还不如多装逼来的要快le很多