a history of science-4-第33章
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nite number of species adapted to peculiarities of soil and climate； and universally distributed； the fact would be in harmony with the idea of special exertion。 But the truth is that various regions exhibit variations altogether without apparent end or purpose。 Professor Henslow enumerates forty…five distinct flowers or sets of plants upon the surface of the earth； notwithstanding that many of these would be equally suitable elsewhere。 The animals of different continents are equally various； few species being the same in any two； though the general character may conform。 The inference at present drawn from this fact is that there must have been； to use the language of the Rev。 Dr。 Pye Smith； 'separate and original creations； perhaps at different and respectively distinct epochs。' It seems hardly conceivable that rational men should give an adherence to such a doctrine when we think of what it involves。 In the single fact that it necessitates a special fiat of the inconceivable Author of this sand…cloud of worlds to produce the flora of St。 Helena； we read its more than sufficient condemnation。 It surely harmonizes far better with our general ideas of nature to suppose that； just as all else in this far…spread science was formed on the laws impressed upon it at first by its Author； so also was this。 An exception presented to us in such a light appears admissible only when we succeed in forbidding our minds to follow out those reasoning processes to which； by another law of the Almighty； they tend； and for which they are adapted。〃＇4＇
Such reasoning as this naturally aroused bitter animadversions； and cannot have been without effect in creating an undercurrent of thought in opposition to the main trend of opinion of the time。 But the book can hardly be said to have done more than that。 Indeed； some critics have denied it even this merit。 After its publication； as before； the conception of transmutation of species remained in the popular estimation； both lay and scientific； an almost forgotten 〃heresy。〃
It is true that here and there a scientist of greater or less reputeas Von Buch； Meckel； and Von Baer in Germany； Bory Saint…Vincent in France； Wells； Grant； and Matthew in England； and Leidy in Americahad expressed more or less tentative dissent from the doctrine of special creation and immutability of species； but their unaggressive suggestions； usually put forward in obscure publications； and incidentally； were utterly overlooked and ignored。 And so； despite the scientific advances along many lines at the middle of the century； the idea of the transmutability of organic races had no such prominence； either in scientific or unscientific circles； as it had acquired fifty years before。 Special creation held the day； seemingly unopposed。
DARWIN AND THE ORIGIN OF SPECIES
But even at this time the fancied security of the special…creation hypothesis was by no means real。 Though it seemed so invincible； its real position was that of an apparently impregnable fortress beneath which； all unbeknown to the garrison； a powder…mine has been dug and lies ready for explosion。 For already there existed in the secluded work…room of an English naturalist； a manuscript volume and a portfolio of notes which might have sufficed； if given publicity； to shatter the entire structure of the special…creation hypothesis。 The naturalist who； by dint of long and patient effort； had constructed this powder…mine of facts was Charles Robert Darwin； grandson of the author of Zoonomia。
As long ago as July 1； 1837； young Darwin； then twenty…eight years of age； had opened a private journal； in which he purposed to record all facts that came to him which seemed to have any bearing on the moot point of the doctrine of transmutation of species。 Four or five years earlier； during the course of that famous trip around the world with Admiral Fitzroy； as naturalist to the Beagle； Darwin had made the personal observations which first tended to shake his belief of the fixity of species。 In South America； in the Pampean formation； he had discovered 〃great fossil animals covered with armor like that on the existing armadillos；〃 and had been struck with this similarity of type between ancient and existing faunas of the same region。 He was also greatly impressed by the manner in which closely related species of animals were observed to replace one another as he proceeded southward over the continent； and 〃by the South…American character of most of the productions of the Galapagos Archipelago； and more especially by the manner in which they differ slightly on each island of the group； none of the islands appearing to be very ancient in a geological sense。〃
At first the full force of these observations did not strike him； for； under sway of Lyell's geological conceptions； he tentatively explained the relative absence of life on one of the Galapagos Islands by suggesting that perhaps no species had been created since that island arose。 But gradually it dawned upon him that such facts as he had observed 〃could only be explained on the supposition that species gradually become modified。〃 From then on； as he afterwards asserted； the subject haunted him； hence the journal of 1837。
It will thus be seen that the idea of the variability of species came to Charles Darwin as an inference from personal observations in the field； not as a thought borrowed from books。 He had； of course； read the works of his grandfather much earlier in life； but the arguments of Zoonomia and The Temple of Nature had not served in the least to weaken his acceptance of the current belief in fixity of species。 Nor had he been more impressed with the doctrine of Lamarck； so closely similar to that of his grandfather。 Indeed； even after his South…American experience had aroused him to a new point of view he was still unable to see anything of value in these earlier attempts at an explanation of the variation of species。 In opening his journal； therefore； he had no preconceived notion of upholding the views of these or any other makers of hypotheses； nor at the time had he formulated any hypothesis of his own。 His mind was open and receptive； he was eager only for facts which might lead him to an understanding of a problem which seemed utterly obscure。 It was something to feel sure that species have varied； but how have such variations been brought about？
It was not long before Darwin found a clew which he thought might lead to the answer he sought。 In casting about for facts he had soon discovered that the most available field for observation lay among domesticated animals； whose numerous variations within specific lines are familiar to every one。 Thus under domestication creatures so tangibly different as a mastiff and a terrier have sprung from a common stock。 So have the Shetland pony； the thoroughbred； and the draught…horse。 In short； there is no domesticated animal that has not developed varieties deviating more or less widely from the parent stock。 Now； how has this been accomplished？ Why； clearly； by the preservation； through selective breeding； of seemingly accidental variations。 Thus one horseman； by constantly selecting animals that 〃chance〃 to have the right build and stamina； finally develops a race of running…horses； while another horseman； by selecting a different series of progenitors； has developed a race of slow； heavy draught animals。
So far； so good； the preservation of 〃accidental〃 variations through selective breeding is plainly a means by which races may be developed that are very different from their original parent form。 But this is under man's supervision and direction。 By what process could such selection be brought about among creatures in a state of nature？ Here surely was a puzzle； and one that must be solved before another step could be taken in this direction。
The key to the solution of this puzzle came into Darwin's mind through a chance reading of the famous essay on 〃Population〃 which Thomas Robert Malthus had published almost half a century before。 This essay； expositing ideas by no means exclusively original with Malthus； emphasizes the fact that organisms tend to increase at a geometrical ratio through successive generations； and hence woul
