Изображения страниц
PDF
EPUB

James Island, of the thirty-eight Galapageian plants, or those found in no other part of the world, thirty are exclusively confined to this one island; and in Albemarle Island, of the twenty-six aboriginal Galapageian plants, twenty-two are confined to this one island, that is, only four are known to grow on the other islands of the Archipelago; and so on, as shown in the above table, with the plants from Chatham and Charles Island."

It is impossible in reading the Origin of Species not to perceive how deeply Mr. Darwin had been impressed by the problems presented by such singularities of plant distribution as he met with in the Galapagos. And of such problems up to the time of its publication no intelligible explanation had seemed possible. Sir Joseph Hooker had indeed prepared the ground by bringing into prominence, in numerous important papers, the no less striking phenomena which were presented when the vegetation of large areas came to be analysed and compared. No one therefore could estimate more justly what Mr. Darwin did for those who worked in this field. How the whole theory of the geographical distribution of plants stood after the publication of

D

the Origin of Species cannot then be better estimated than from the summary of the position, contained in Sir Joseph Hooker's recent Address to the Geographical Section of the meeting of the British Association at York.

"Before the publication of the doctrine of the origin of species by variation and natural selection, all reasoning on their distribution was in subordination to the idea that these were permanent and special creations; just as, before it was shown that species were often older than the islands and mountains they inhabited, naturalists had to make their theories accord with the idea that all migration took place under existing conditions of land and sea. Hitherto the modes of dispersion of species, genera, and families had been traced, but the origin of representative species, genera, and families, remained an enigma; these could be explained only by the supposition that the localities where they occurred presented conditions so similar that they favoured the creation of similar organisms. But this failed to account for representation occurring in the far more numerous cases where there is no discoverable similarity of physical conditions, and

of their not occurring in places where the conditions are similar. Now under the theory of modification of species after migration and isolation, their representation in distant localities is only a question of time and changed physical conditions. In fact, as Mr. Darwin well sums up, all the leading facts of distribution are clearly explicable under this theory; such as the multiplication of new forms, the importance of barriers in forming and separating zoological and botanical provinces; the concentration of related species in the same area; the linking together under different latitudes of the inhabitants of the plains and mountains, of the forests, marshes, and deserts, and the linking of these with the extinct beings which formerly inhabited the same areas; and the fact of different forms of life occurring in areas having nearly the same physical conditions."

If Mr. Darwin had done no more than this for botanical science he would have left an indelible mark on its progress. But the consideration of the various questions which the problem of the origin of species presented led him into other inquiries in which the results were scarcely less important. The key-note of a whole series of his writings is struck

by the words with which the eighth chapter of the Origin of Species commences :—

“The view generally entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent the confusion of all organic forms."

The examination of this principle necessarily obliged him to make a profound study of the conditions and limits of sterility. The results embodied in his well-known papers on dimorphic and trimorphic plants afforded an absolutely conclusive proof that sterility was not inseparably tied up with specific divergence. But the question is handled in the most cautious way, and when the reader of the chapter on hybridism arrives at the concluding words, in which Mr. Darwin declares that on this ground "there is no fundamental distinction between species and varieties," he finds himself in much the same intellectual position as is produced by the Q.E.D. at the end of a geometrical demonstration.

It was characteristic of Mr. Darwin's method of study to follow up on its own account, as completely as possible, when opportunity presented, any side issue which had been raised apparently incidentally

in other discussions.

Indeed, it was never possible

to guess what amount of evidence Mr. Darwin had in reserve behind the few words which marked a mere step in an argument. It is from his practice of bringing out from time to time the contents of his unseen treasure-house that we gain some insight into the scientific fertility of his later years, at first sight so inexplicably prolific. Many of his works published during that period may be properly regarded in the light of disquisitions on particular points of his great theory. The researches on the sexual phenomena of heterostyled plants, alluded to above, which were communicated to the Linnean Society in a series of papers ranging over the years 1862-8, ultimately found their complete development in the volume on the Different Forms of Flowers on Plants of the same Species, published in 1877. In the same way, the statement in the Origin of Species, that "the crossing of forms only slightly differentiated favours the vigour and fertility of their offspring," finds its complete expansion in The Effects of Cross and Self-Fertilisaton in the Vegetable Kingdom, published in 1876.

The Origin of Species in the form in which

« ПредыдущаяПродолжить »