SUPPLEMENT.

The following additions and alterations, prepared by the author expressly for this edition, were received too late to be incorporated in their proper places.

Page 46, eight lines from bottom, after "not generally propagated," insert:

If it could be shown that monstrosities were even propagated for a succession of generations in a state of nature, modifications might be effected (with the aid of natural selection) more abruptly than I am inclined to believe they are.

Page 79, six lines from bottom, after word "nature," insert parenthesis-reading the whole sentence thus:

Man can act only on external and visible characters: nature (if I may be allowed for brevity-sake to personify the natural preservation of favoured individuals during the struggle for existence) cares nothing for appearances, except in so far as they may be useful to any being.

Page 168, first line, after "structure of the eye," insert: (though in the fish Amphioxus, the eye is in an extremely simple condition without a lens)

Page 168, sixth line from top, omit all seventeen lines beginning with In the Articulata," and ending with "living crustaceans, and bearing in mind," and insert as follows:

[In the great kingdom of the Articulata we can start

from an optic nerve, simply coated with pigment, which sometimes forms a sort of pupil, but is destitute of a lens or any other optical mechanism. From this rudimentary eye which can distinguish light from darkness, but nothing else, there is an advance towards perfection along two lines of structure, which Müller thought were fundamentally different; namely, firstly, stemmata, or the so-called "simple eyes," which have a lens and cornea; and secondly, "compound eyes," which seem to act mainly or solely by excluding all the rays from each point of the viewed object, except the pencil that comes in a line perpendicular to the convex retina. In the class of compound eyes, besides endless differences in the form, proportion, number and position of the transparent cones coated by pigment which act by exclusion, we have additions of a more or less perfect concentrating apparatus; thus in the eyes of the meloe the facets of the cornea are "slightly convex both externally and internally; that is, lens-shaped." In many crustaceans there are two cornea, the external smooth and the internal divided into facets, within the substance of which, as Milne Edwards says, "renflemens lenticulaires paraissent s'etre développés;" and sometimes these lenses can be detached in a layer distinct from the cornea. The transparent cones, which were supposed by Müller to act solely by excluding the divergent pencils of light, usually adhere to the cornea, but not rarely they are detached from it, and have their free ends convex, and in this case they must act, I presume, as converging lenses. Altogether, so diversified is the structure of the compound eyes, that Müller makes three main classes, with no less than seven subdivisions of structure. He makes a fourth main class-namely, "aggregates" of stemmata; and he adds that "this is the transition-form between the mosaic-like compound eyes, unprovided with a concentrating apparatus and organs of vision with such an apparatus."

With these facts, here too briefly and imperfectly given, which show how much graduated diversity there is in the eye of our existing articulata, and bearing in mind, &c., &c.

Page 169, tenth line from top, after "transparent tissue," add:

with spaces filled with fluid and with a nerve sensitive, &c., &c.

Page 169, sixteenth line from top, after "there is a power," insert in brackets:

[natural selection]

Page 170, twenty-first line from top, after "insensible steps," insert:

Certain plants, as some Leguminosa, Violaceæ, &c., bear two kinds of flowers; one having the normal structure of the order, the other kind being degraded, though sometimes more fertile than the perfect flowers; if the plant ceased to bear its perfect flowers, and this did occur during several years with an imported specimen of Aspicarpa in France, a great and sudden transition would in fact be effected in the nature of the plant.

Pages 293 and 294. Omit thirty lines, beginning, "On the state of Development," and ending with "class, may have beaten the highest molluscs," and insert as follows:

On the state of Development of ancient compared with living Forms. We have seen in the Fourth Chapter that the degree of differentiation and specialisation of the parts of all organic beings, when come to maturity, is the best standard as yet suggested of their degree of perfection or highness. We have also seen that, as the specialisation of parts or organs is an advantage to each being, so natural selection will constantly tend thus to render the organisation of each more specialised and perfect, and in this sense higher; not but that it may, and will, leave many creatures fitted for simple conditions of life with simple and unimproved structures. In another and more general manner we can see that, on the theory of natural selection, the more recent forms will tend to be higher than their progenitors; for each new species is formed by having had some advantage in the struggle for life over other and preceding forms. If, under a nearly similar

climate, the eocene inhabitants of one quarter of the world were put into competition with the existing inhabitants of the same or some other quarter, the eocene fauna or flora would certainly be beaten and exterminated; as would a secondary fauna by an eocene, and a palæozoic fauna by a secondary fauna. So that by this fundamental test of victory in the battle for life, as well as by the standard of the specialisation of organs, modern forms ought, on the theory of natural selection, to stand higher than ancient forms. Is this the case? A large majority of palæontologists would certainly answer in the affirmative; but in my imperfect judgment, I can, after having read the discussions on this subject by Lyell and by Hooker in regard to plants, concur only to a cretain limited extent. Ciri Nevertheless, it may be anticipated that the evidence will be rendered more decisive by future geological research.

The problem is in many ways excessively intricate. The geological record, at all times imperfect, does not extend far enough back, as I believe, to show with unmistakable clearness that within the known history of the world organisation has largely advanced. Even at the present day, looking to members of the same class, naturalists are not unanimous which forms are highest; thus some look at the Selaceans or sharks from their approach in some important point of structure to reptiles as the highest fish; others look at the teleosteans as the highest. The ganoids stand intermediate between the selaceans and teleosteans; the latter, at the present day, are largely preponderant in number, but formerly selaceans and ganoids alone existed; and in this case, according to the standard of highness chosen, so will it be said that fishes have advanced or have retrograded in organisation. To attempt to compare in the scale of highness members of distinct types seems hopeless; who will decide whether a cuttle-fish be higher than a bee? that insect which the great Von Baer believed to be "in fact more highly organised than a fish, although upon another type." In the complex struggle for life it is quite credible that crustaceans, for instance, not very high in their own class, might beat the cephalopods or highest molluscs; and such crus

taceans, though not highly developed, would stand very high in the scale of invertebrate animals, if judged by the most decisive of all trials,-the law of battle.

Besides this inherent difficulty in deciding which forms are the most advanced in organization, we ought not solely to compare the highest members of a class at any two distant periods,-though undoubtedly this is one and perhaps the most important element in striking a balance -but we ought to compare all the members, high and low, at the two periods. At an ancient epoch the highest and lowest molluscs-namely, cephalopods and brachiopods-swarmed in numbers; at the present time, both these orders have been greatly reduced, whereas other orders, intermediate in grade of organisation, have largely increased; consequently some naturalists have maintained that molluscs were formerly more highly developed than at present; but a stronger case can be made out on the other side by considering the vast reduction at the present day of the lowest molluscs; more especially as the exist ing cephalopods, though so few in number, are more highly organised than their ancient representatives. We ought, also, to consider the relative proportional numbers of the high and low classes in the population of the world at the two periods; if, for instance, at the present day there were fifty thousand kinds of vertebrate animals, and if we had reason to believe that at some former period there were only ten thousand kinds, we ought to look at this increase in number of the highest class, which implies a great displacement of lower forms, as a decided advance in the organisation of the world, whether or not it were the higher vertebrata which had thus largely increased. We can thus see how hopelessly difficult it will apparently forever be to compare with perfect fairness under such extremely complex relations the standard of organisation of the imperfectly known faunas of successive periods of the earth's history.

We shall appreciate (under one important point of view) this difficulty the more clearly, by looking to the case of certain existing faunas and floras. From the extraordinary manner in which European productions have recently spread over New Zealand, &c., &c.