登陆注册
15713900000155

第155章

To refer once again to cirripedes: the larvae in the first stage have three pairs of legs, a very simple single eye, and a probosciformed mouth, with which they feed largely, for they increase much in size. In the second stage, answering to the chrysalis stage of butterflies, they have six pairs of beautifully constructed natatory legs, a pair of magnificent compound eyes, and extremely complex antennae; but they have a closed and imperfect mouth, and cannot feed: their function at this stage is, to search by their well-developed organs of sense, and to reach by their active powers of swimming, a proper place on which to become attached and to undergo their final metamorphosis. When this is completed they are fixed for life: their legs are now converted into prehensile organs; they again obtain a well-constructed mouth; but they have no antennae, and their two eyes are now reconverted into a minute, single, and very simple eye-spot. In this last and complete state, cirripedes may be considered as either more highly or more lowly organised than they were in the larval condition. But in some genera the larvae become developed either into hermaphrodites having the ordinary structure, or into what I have called complemental males: and in the latter, the development has assuredly been retrograde; for the male is a mere sack, which lives for a short time, and is destitute of mouth, stomach, or other organ of importance, excepting for reproduction.

We are so much accustomed to see differences in structure between the embryo and the adult, and likewise a close similarity in the embryos of widely different animals within the same class, that we might be led to look at these facts as necessarily contingent in some manner on growth.

But there is no obvious reason why, for instance, the wing of a bat, or the fin of a porpoise, should not have been sketched out with all the parts in proper proportion, as soon as any structure became visible in the embryo.

And in some whole groups of animals and in certain members of other groups, the embryo does not at any period differ widely from the adult: thus Owen has remarked in regard to cuttle-fish, `there is no metamorphosis; the cephalopodic character is manifested long before the parts of the embryo are completed;' and again in spiders, `there is nothing worthy to be called a metamorphosis.' The larvae of insects, whether adapted to the most diverse and active habits, or quite inactive, being fed by their parents or placed in the midst of proper nutriment, yet nearly all pass through a similar worm-like stage of development; but in some few cases, as in that of Aphis, if we look to the admirable drawings by Professor Huxley of the development of this insect, we see no trace of the vermiform stage.

How, then, can we explain these several facts in embryology, namely the very general, but not universal difference in structure between the embryo and the adult; of parts in the same individual embryo, which ultimately become very unlike and serve for diverse purposes, being at this early period of growth alike; of embryos of different species within the same class, generally, but not universally, resembling each other; of the structure of the embryo not being closely related to its conditions of existence, except when the embryo becomes at any period of life active and has to provide for itself; of the embryo apparently having sometimes a higher organisation than the mature animal, into which it is developed. I believe that all these facts can be explained, as follows, on the view of descent with modification.

It is commonly assumed, perhaps from monstrosities often affecting the embryo at a very early period, that slight variations necessarily appear at an equally early period. But we have little evidence on this head indeed the evidence rather points the other way; for it is notorious that breeders of cattle, horses, and various fancy animals, cannot positively tell, until some time after the animal has been born, what its merits or form will ultimately turn out. We see this plainly in our own children; we cannot always tell whether the child will be tall or short, or what its precise features will be. The question is not, at what period of life any variation has been caused, but at what period it is fully displayed. The cause may have acted, and I believe generally has acted, even before the embryo is formed; and the variation may be due to the male and female sexual elements having been affected by the conditions to which either parent, or their ancestors, have been exposed. Nevertheless an effect thus caused at a very early period, even before the formation of the embryo, may appear late in life; as when an hereditary disease, which appears in old age alone, has been communicated to the offspring from the reproductive element of one parent. Or again, as when the horns of cross-bred cattle have been affected by the shape of the horns of either parent. For the welfare of a very young animal, as long as it remains in its mother's womb, or in the egg, or as long as it is nourished and protected by its parent, it must be quite unimportant whether most of its characters are fully acquired a little earlier or later in life. It would not signify, for instance, to a bird which obtained its food best by having a long beak, whether or not it assumed a beak of this particular length, as long as it was fed by its parents. Hence, I conclude, that it is quite possible, that each of the many successive modifications, by which each species has acquired its present structure, may have supervened at a not very early period of life; and some direct evidence from our domestic animals supports this view. But in other cases it is quite possible that each successive modification, or most of them, may have appeared at an extremely early period.

同类推荐
热门推荐
  • 一方通行屠神录

    一方通行屠神录

    为了拯救某人,为了向某些人赎罪,一方通行不幸穿越到地下城。本以为可以凭借实力碾压一切,却不知……“呐,呐,一方通行你总是最强最强的这么自恋的称赞自己,其实……这只是你自己妄想的吧。”“哈?!开什么玩笑!”“那么最强什么的,究竟是多强啊,不会只是听着有些酷吧。”听到少女的询问,一方通行不屑一笑,抬起苍白瘦弱的单臂指着淡蓝色的天空如此狂妄的说道,“如果这天上有神……老子能屠了他!”
  • 总裁,别太娇宠

    总裁,别太娇宠

    世间是不是真的有因果?才会让你我经过上一辈的恩怨,再在这里遇见。有女小白,迷糊无双,有子炎凉,郎才艳艳。爱情从来没有坚定不移的,只有你及时出击,它才可能为你停留。爱情不是随心所欲的,不是你说有就有的,以前,一颗真心摆在你的面前,是你躲避不及,可是如今她已经放下,你为什么又开始纠缠?本文就是一个小白兔遇上大灰狼的故事,本文绝对不虐,有的只是甜蜜(^_^)求亲们多多收藏,点赞,打赏呗!推荐晓晓的其他红袖文!!!
  • 捡到只系统君肿么破

    捡到只系统君肿么破

    系统,明星文女猪脚:自闭症患者自捡到系统君后往吐槽的道路越走越远系统君:不卖萌会死星人性别不明总的说,这是一篇女主捡到喵星人一只后在喵星人的帮助下走向巨星之路.拜脱自闭症的成长文,所以没有男主是很正常的(本作者才不会承认是写爱情线无能的原因呢!)因此,读者们注意了啊,想看爱情的慎入!但是呢额……本作者如果心情好突然EQ爆涨也可能“不小心”爆出顺舒的爱情线哦~~敬请期待!
  • 爱在向西行

    爱在向西行

    用一句话说,她很拽,特别拽,拽得他有时恨不能掐死她。可更多的时候他却更喜欢她那拽拽的样子,带点白目,特可爱。
  • 邪后太妖娆

    邪后太妖娆

    某女一个媚眼,手指朝面前的人勾了勾,邪笑道“过来,让我亲一口~”某男淡淡撇了某女一眼“就这么饥渴?”某女“吼~谁让你长得这么帅呢,一见到你我就把持不出嘞~”某男“怪我咯。”某女“矮油,不要这么冷淡嘛”某男“对不起天生的”某女“切,装什么,再怎么禁欲不还是让我勾搭到了~”某男“勾引你侄女的驸马,你果真不要脸……”某女捂脸“矮油,瞎说什么大实话!”某男“……”某女“话说……你她妈过不过来!”
  • 动心魔法

    动心魔法

    年轻貌美的于子鱼患有严重的公主病,觉得世间的男子少有能配上自己的。转眼30岁,岁月这把杀猪刀给她狠狠的上了一课。看着身边越来越多年轻粉嫩的网红美女,她早对自己的终身大事不抱有任何幻想。直到一个寒冷的冬夜,她掉入了一个很深的雪坑,就在她以为自己小命不保的时候,竟然意外的跌进了一个游戏界面,获得了一瓶药水,她的生活轨迹完全的改变了......
  • 冰蝶泪雨冰心恋

    冰蝶泪雨冰心恋

    她,冷漠无情,如同冰山,却愿为她(他)们而死。她,温柔可人,如同清水,却愿为她手染鲜血。她,火热张扬,如同孩子,却愿为她一人成熟细心。她受伤离开了爱她的家人,出国留学,遇到她们。她回归故国,遇见他们,他无情淡薄,却唯独对她一个人温柔;他,冷漠淡薄,却为她一人担忧,遇到和她一同归来的她;他,活泼花心,却为她一人成熟,他们几人的故事即将开始,请看文章。第一次写文,请多多关照,支持,指教。我会更完这一本书,如果有几天漏更,请多担待
  • 上清狐狸

    上清狐狸

    封神之战后,截教为天道所弃,圣人之下皆蝼蚁,凡修炼上清仙法者必遭天罚,幽禁于三十三外天的通天圣人有一天突然感应到,人间一只狐妖得到了他的传承……胡小玉穿成玉面狐狸后,为了摆脱牛魔王,努力修炼之余,作为狐狸精的她,还坚定地抱上了牛魔王的主子——通天圣人的大腿!--情节虚构,请勿模仿
  • 守护甜心之冰山复仇

    守护甜心之冰山复仇

    以前那个活泼的日奈森亚梦已经不复存在留下的只是一个傀儡班的雪翼紫陌,她为了最后的复仇,仅仅只剩下了一个个躯壳,他是多么的无助呀!一起来接受她的挑战吧!!
  • 陌桑纤尘舞

    陌桑纤尘舞

    汉月无疆酒杯里堆积如雪的月色,照进梦中流转千年的壮丽山河。那场湮灭多年的铁马金戈,只剩一曲泪断情肠的千古离歌。英雄埋骨的大漠长河,不让须眉的姑娘可愿做一回红尘过客?万里黄沙掩不住心如刀割,杯酒柔情都化做无人清扫的爱恨纠葛。是不是用前生演一场悲欢离合,来世就会懂得什么叫做失而复得?