Изображения страниц
PDF
EPUB

common shell like Spirifera striata, we find two difficulties-one practical-arising from the want of an adequate public collection, to which reference might always be made, for foreign as well as British fossils, for varieties of race, and examples of the young and the aged, and of stunted, deformed, or gigantic growth. The best figures and the best descriptions will never suffice for determining specific distinctions in particular cases of very variable groups. This practical difficulty, of becoming perfectly acquainted with the accepted nomenclature, is aggravated by the cloud of theoretical obscurity which has been for some time condensing round the origin and character of species. Are the groups which we call species united by characters which vary only within definable limits? Are these limits of variation for a given species constant for all the area of its province, and for all the period of its geological range? Palæontologists generally answer in the affirmative, and the type-specimens in our Museums appear to confirm it. But if instead of these selections to exemplify differences we make large collections to illustrate agreements, the species (so-called) seem to lose their distinctness, and a second, or even a third or fourth group of forms may be fairly united to the first, before arriving at a real circumscription available for space and for time.

This process, in the careful hands of Mr Davidson, has already reduced the number of species in several groups of Brachiopoda; and augmented the geographical area of the groups. When fully carried out, I expect it will be found that the Brachiopoda of the Carboniferous limestones, so remarkably allied over all Europe, have a much larger agreement with those of North America than is at present allowed in our Catalogues.

In this process of reducing the admitted number of species we are following in the track of great botanists like Bentham and Hooker, whose researches in modern nature have brought them into view of similar difficulties.

But what if, as naturalists of no mean fame have told us, the limits we thus draw round species are arbitrary functions of our own minds, not real boundaries set by nature; limits which are not chosen alike by different minds, nor surely and firmly retained by the same mind, at different times? If such an opinion, whether true or not, were accepted by physiologists from a survey of existing nature, how would it affect our view of the succession of life on the globe? Those who maintain it to be true, usually assume some hypothesis which involves the operation of long time, and thus brings the question within the judgment of Geology.

NATURAL SELECTION.

Moved by such considerations, an eminent naturalist and geologist of our day has quitted the high road of the Systema Natura, and is seeking for the history of species' by a path which takes the same general direction as that already explored by Lamarck toward the primitive germs and primordial forms of life. Nature, he affirms, in successive generations gives varieties; these in the struggle for existence have unequal fortune, those most adapted to the circumstances of the time and place prosper, and give origin to descendants which run the same risks, and under the same principle of 'natural selection' acquire more and more the character of distinctness and of superiority. Following out these ideas, he has arrived at results which are thus expressed:

'Although much remains obscure, and will long remain obscure, I can entertain no doubt, after the most deliberate study and most dispassionate judgment of which I am capable, that the view which most naturalists entertain, and which I formerly entertained, namely, that each species has been independently created, is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera

1 Darwin, Origin of Species, 1859.

are lineal descendants of some other and generally extinct species, in the same way as the acknowledged varieties of any one species are the descendants of that species. Furthermore, I am convinced that natural selection has been the main but not the exclusive means of modification.'

If asked how far he extends the doctrine of the mutability of species, he replies:

"The question is difficult to answer, because the more distinct the forms are which we may consider, by so much the arguments fall away in force. But some arguments of the greatest weight extend very far. All the members of whole classes can be connected together by chains of affinities, and all can be classified on the same principle, in groups subordinate to groups. Fossil remains sometimes tend to fill up very wide intervals between existing orders. Organs in a rudimentary condition plainly show that an early progenitor had the organs in a fully developed state; and this in some instances necessarily implies an enormous amount of modification in the descendants. Throughout whole classes various structures are formed on the same pattern, and at an embryonic age the species closely resemble each other. Therefore I cannot doubt that the theory of descent with modification embraces all the members of the same class. I believe that animals have

descended from at most only four or five progenitors, and plants from an equal or lesser number.

'Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from one prototype. But analogy may be a deceitful guide. Nevertheless all living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure, and their laws of growth and reproduction. We see this even in so trifling a circumstance as that the same poison often similarly affects plants and animals; or that the poison secreted by the gall-fly produces monstrous growths on the wild rose or oak-tree. Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form into which life was first breathed.

'As all the living forms of life are the lineal descendants of those which lived long before the Silurian epoch, we may feel certain that the ordinary succession by generation has never once been broken, and that no cataclysm has desolated the whole world. Hence we may look with some confidence to a secure future of equally inappreciable length. And as natural selection works solely by and for the good of each being, all corporeal and mental endowments will tend to progress toward perfection.

« ПредыдущаяПродолжить »