Heredity in simple forms of life—In more complex organisms—Pangenesis—Varieties how produced—Fixed1 by law of survival of the fittest—Dr. Temple’s view—Examples: triton, axolotl—Variations in individuals and species—Lizards into birds—Ringed snakes—Echidna.
As the earth is kept in an orbit, which makes life possible by the balance of the antagonist3 centripetal4 and centrifugal forces, so is that life evolved and maintained by the balance of the two conflicting forces of heredity and variation. Heredity, or the principle which makes offsprings resemble their parental5 organisms, may be considered as the centripetal force which gives stability to species; while variation is like the centrifugal force which tends to make them develop into new forms, and prevents organic matter from remaining ever consolidated6 into one uniform mass.
As regards heredity, the considerations which have been advanced in the last chapter, on the origin of sex, will enable the reader to understand the principles on which it is based. When a moneron, or living piece of pure protoplasm, or its successor the nucleated cell, propagates itself by simple division into two equal parts, it is obvious that each half must, in its atomic constitution and motions, exactly resemble the original. If am?ba A divides into am?b? B and C, both B and C[118] are exact facsimiles of A and of one another, and so are the progeny8 of B and C through any number of generations. They must remain identical repetitions of the parent form, unless some of them should happen to be modified by different actions of their surrounding environment, powerful enough to affect the original organisation9.
In propagation by germs or buds, the same thing must hold true, only, as the offspring carries with it not the half, but only a small portion of the parental organism, its impress will be less powerful, and the new organism will more readily be affected10 by external influences. When we come to propagation by spores11 or single cells, and still more to sexual propagation by the union of single cells of two progenitors12, it becomes more difficult to see how the type of the two parents, and of a long line of preceding ancestors, can be maintained so perfectly13.
Of the fact that it is maintained there can be no doubt. Not only do species breed true and remain substantially the same for immense periods, but the characters of individual parents and their ancestors repeat themselves, to a great extent, in their offspring. Thus the cross between the white and black varieties of the human species perpetuates14 itself to such an extent, that a single cross of black blood leaves traces for a number of generations. In the Spanish American States and the West Indies, where the distinction is closely observed, the term ‘octoroon’ is well known, as applied15 to Creoles who have seven-eighths of white to one-eighth of black blood in their composition. In the case of what is called ‘atavism,’ this recurrence16 to the characters of ancestors is carried to a much further[119] extent. In breeding animals, it is not uncommon17 to find the peculiar18 features of generations of ancestors long since extinct cropping up occasionally in individuals. Thus, stripes like those of the ass7 along the back and down the shoulders, occasionally appear on horses whose immediate19 ancestors for many generations back showed nothing of the sort; and even stripes across the legs like those of the zebra occur quite unexpectedly, and testify to the common descent of the various species of the horse tribe from a striped ancestor. How these ancestral peculiarities20 can be transmitted through many generations, each individual of which originated from a single microscopic21 cell which had been fructified22 by another cell, is one of the greatest mysteries of nature. It may assist us in forming some idea of the possibility of a solution to remember what has been proved as to the dimensions of atoms. Their order of magnitude is that of a cricket-ball to the earth. In a single microscopic cell, therefore, there may be myriads23 of such atoms circling round one another and forming infinitesimal solar systems, of infinite complexity24 and variety. Darwin’s theory of ‘Pangenesis’ supposes that some of the actual identical atoms which formed part of ancestral bodies are thus transmitted through their descendants for generation after generation, so that we are literally25 ‘flesh of the flesh’ of the prim26?val creature who was developed into man in the later tertiary or early glacial period. Haeckel, more plausibly27, suggests that not the identical atoms, but their peculiar motions and mode of aggregation28 have been thus transmitted: a mode of transmission which, with his prevailing29 tendency to invent long and learned names for everything, he calls the[120] ‘Perigenesis of plastids.’ In any case, however, these must be taken not as solutions of the problem, but as guesses at the truth which show that its solution is not impossible.
The opposite principle to heredity, that of variation, is equally important and universal. It is apparent in the fact, that although every individual of every species reproduces qualities of parents and ancestors, no two individuals do so in precisely30 the same manner; no two are exactly alike. This difference, or individuality, becomes more marked as the organism is higher. Thus, sheep and hounds differ from one another by slight differences which require the practised eye of the shepherd or huntsman to detect; while human beings are so unlike, that of the many millions existing in each generation no two exactly resemble one another. The reason of this is apparent if we consider that the higher the organism the more complex does it become, and the less the chance of the whole complicated relations of parent and ancestral organisms being transmitted by single cells so solidly and completely as to overpower and remain uninfluenced by external influences. Variation evidently depends mainly on the varying influences of environment. If the exterior31 layer of molecules33 of a lump of protoplasm become differentiated34 from the interior ones and form a cell-wall, it is because they are in more immediate contact with the air or other surrounding medium. Internal changes depend on conditions such as temperature and nutrition. In the case of cultivated plants and domestic animals we can see most clearly how varieties are produced by adaptation to changes of environment. These variations, however, would not proceed very far, were it not for the interaction[121] of the opposing forces of variation and heredity, by which latter the variations appearing in individuals are fixed and accumulated in descendants, until they become wide and permanent divergencies. This is done in the case of cultivated plants and domestic animals by man’s artificial selection in pairing individuals who show the same variations; and in nature by the struggle for existence, giving victory and survival to those forms, and in the long run to those forms only, whose variations, slight as they may be in each generation, tend to bring individuals into better adaptation to their environment.
It is the great glory of Darwin to have established this firmly by an immense number of interesting and exhaustive instances, and thus placed evolution, or a scientific explanation of the development and laws of life, on a solid basis. Every day fresh discoveries and experiments confirm this great principle, and it has almost passed into the same phase as Newton’s law of gravity, as a fundamental law accepted as axiomatic35 by all men of science, and as the basis of modern thought, to which all religions and philosophies have to conform, accepted by nearly all modern thinkers. I may here quote a passage from an eminent36 Anglican divine, Dr. Temple, for the double purpose of showing how universal has become the acceptance of this Darwinian view of evolution among intelligent men; and how little terrible are its consequences, even to those who look at the facts of the universe through a theological medium and retain their belief in accepted creeds37.
‘It seems in itself something more majestic38, more befitting of Him to whom a thousand years are as one day, and one day as a thousand years, thus to impress His will once for all on this creation, and provide for[122] all its countless39 varieties by this one original impress, than by special acts of creation to be perpetually modifying what He had previously40 made.’[1]
[1] Dr. Temple, Religion and Science.
Scientific men would be content to accept this statement of Dr. Temple’s almost in his own words, except that they might consider his definition of the Great First Cause as somewhat too absolute and confident. Having had to deal so much with actual facts and accurate knowledge, they are apt to be more modest in assertion than even the most enlightened theologian, whose studies have lain rather in the direction of phrases and ideas, which, from their very nature, are more vague and indefinite, and perhaps rather guesses and aspirations41 after truth, than proofs of it. In any case there is the authority of a learned and liberal-minded bishop42 for the position that the scientific way of looking at the universe is not necessarily profane43 or irreligious.
To return to variation: the instances of the operation of this principle, alone or in conjunction with that of heredity, in working out the evolution of species, are exceedingly numerous and interesting. Those who wish to understand the subject thoroughly44 must study the works of Darwin, Haeckel, Huxley, and other modern writers; but for my present purpose it will be sufficient to refer to a few of the most marked instances which may assist the reader in comprehending how the gradual evolution of life and creation of new species may have been brought about.
There is an amphibious animal, called the triton or water-salamander, akin45 to the frog, whose normal course is to begin life living in the water and breathing by gills, and end it on land with gills metamorphosed into[123] lungs. If they are shut up in water and kept in a tank they never lose their gills, but continue through life in the lower stage of development, and reproduce themselves in other tritons with gills. Conversely the axolotl, a peculiar gilled salamander from the Lake of Mexico, has its normal course to live, die, and propagate its species in water, breathing by gills; but if an axolotl happens to stray from the water and take to living on dry land, the gills are modified into lungs and the animal gains a place in the class in the school of development. This fits in remarkably46 with the fact that the embryo47 of all vertebrate mammals, including man, passes through the gilled stage before arriving at the development of lungs, which assists us in understanding two facts of primary importance in the history of evolution.
First, how terrestrial life may have arisen from aquatic48 life by adaptation to altered conditions.
Secondly49, how the evolution of the embryo sums up in the individual, in the period of a few days or months, the various stages of evolutions which it has taken millions of years to accomplish in the species.
As a parallel to the transformation50 of gills into lungs, and of an aquatic into a land animal, if we turn to the geological records of the Secondary period we may trace the transformation of a water into an air population, of sea-lizards into flying-lizards, and of flying-lizards into birds. The ‘Hesperornis’ is an actual specimen51 of the transition, being a feathered lizard2, or rather winged and feathered creature which is half lizard and half bird.
A remarkable52 instance of the great change of functions which may be produced by a change of outward[124] conditions is afforded by the common ringed snake, which in its natural state lays eggs which take three weeks to hatch; but if confined in a cage in which no sand is strewed53 it hatches the eggs within its own body, and from oviparous becomes viviparous. This may help us to understand how the lowest order of mammals, which, like the Australian echidna or duck-billed mole32, lay eggs, may have developed, first into marsupial54, and finally into placental mammals.
These examples may assist the reader in understanding how the infinite diversities of living species may have been developed in the course of evolution from simple origins, just as the inorganic55 world was from atoms, by the action and reaction of primitive56 polar forces between the organism and its environment, and between heredity and variation.
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1
fixed
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| adj.固定的，不变的，准备好的；(计算机)固定的 | |
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2
lizard
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| n.蜥蜴，壁虎 | |
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3
antagonist
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| n.敌人，对抗者，对手 | |
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centripetal
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| adj.向心的 | |
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parental
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| adj.父母的；父的；母的 | |
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6
consolidated
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| a.联合的 | |
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ass
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| n.驴；傻瓜，蠢笨的人 | |
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progeny
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| n.后代，子孙；结果 | |
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organisation
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| n.组织，安排，团体，有机休 | |
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affected
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| adj.不自然的，假装的 | |
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spores
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| n.（细菌、苔藓、蕨类植物）孢子( spore的名词复数 )v.（细菌、苔藓、蕨类植物）孢子( spore的第三人称单数 ) | |
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12
progenitors
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| n.祖先( progenitor的名词复数 )；先驱；前辈；原本 | |
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perfectly
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| adv.完美地，无可非议地，彻底地 | |
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14
perpetuates
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| n.使永存，使人记住不忘( perpetuate的名词复数 )；使永久化，使持久化，使持续 | |
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applied
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| adj.应用的；v.应用，适用 | |
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recurrence
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| n.复发，反复，重现 | |
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uncommon
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| adj.罕见的，非凡的，不平常的 | |
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18
peculiar
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| adj.古怪的，异常的；特殊的，特有的 | |
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immediate
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| adj.立即的；直接的，最接近的；紧靠的 | |
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peculiarities
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| n. 特质, 特性, 怪癖, 古怪 | |
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microscopic
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| adj.微小的，细微的，极小的，显微的 | |
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fructified
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| v.结果实( fructify的过去式和过去分词 )；使结果实，使多产，使土地肥沃 | |
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23
myriads
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| n.无数，极大数量( myriad的名词复数 ) | |
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24
complexity
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| n.复杂(性)，复杂的事物 | |
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literally
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| adv.照字面意义，逐字地；确实 | |
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prim
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| adj.拘泥形式的，一本正经的；n.循规蹈矩，整洁；adv.循规蹈矩地，整洁地 | |
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plausibly
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| 似真地 | |
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28
aggregation
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| n.聚合，组合；凝聚 | |
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prevailing
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| adj.盛行的；占优势的；主要的 | |
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precisely
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| adv.恰好，正好，精确地，细致地 | |
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31
exterior
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| adj.外部的，外在的；表面的 | |
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32
mole
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| n.胎块；痣；克分子 | |
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33
molecules
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| 分子( molecule的名词复数 ) | |
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34
differentiated
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| 区分，区别，辨别( differentiate的过去式和过去分词 )； 区别对待； 表明…间的差别，构成…间差别的特征 | |
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35
axiomatic
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| adj.不需证明的，不言自明的 | |
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eminent
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| adj.显赫的，杰出的，有名的，优良的 | |
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creeds
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| （尤指宗教）信条，教条( creed的名词复数 ) | |
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majestic
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| adj.雄伟的，壮丽的，庄严的，威严的，崇高的 | |
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countless
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| adj.无数的，多得不计其数的 | |
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40
previously
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| adv.以前，先前（地） | |
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41
aspirations
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| 强烈的愿望( aspiration的名词复数 )； 志向； 发送气音； 发 h 音 | |
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42
bishop
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| n.主教，（国际象棋）象 | |
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43
profane
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| adj.亵神的，亵渎的；vt.亵渎，玷污 | |
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44
thoroughly
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| adv.完全地，彻底地，十足地 | |
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45
akin
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| adj.同族的，类似的 | |
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46
remarkably
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| ad.不同寻常地，相当地 | |
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47
embryo
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| n.胚胎，萌芽的事物 | |
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48
aquatic
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| adj.水生的，水栖的 | |
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49
secondly
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| adv.第二，其次 | |
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50
transformation
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| n.变化；改造；转变 | |
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51
specimen
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| n.样本，标本 | |
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52
remarkable
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| adj.显著的，异常的，非凡的，值得注意的 | |
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53
strewed
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| v.撒在…上( strew的过去式和过去分词 )；散落于；点缀；撒满 | |
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54
marsupial
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| adj.有袋的，袋状的 | |
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55
inorganic
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| adj.无生物的；无机的 | |
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56
primitive
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| adj.原始的；简单的；n.原(始)人，原始事物 | |
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