occupies several pages of his Darwin and After-Darwin with correcting what he regards as the mistakes and fallacies concerning Natural Selection of which its advocates no less than its opponents are guilty. Certainly, it must be admitted that some very shrewd and clever persons have engaged in this interesting enquiry, and it is therefore startling to find that it is so easy to fall into error as to what the theory means or implies.

This liability to misapprehension, however, is not altogether the fault of the critic or of the student; for putting on one side those points in which one expert differs from another, such as whether Natural Selection is the sole method of the transmutation of species, or only one among many, we have to take note of the fact that one writer sometimes contradicts himself on questions of very great importance-such as the nature of the variations from which selection is made; the strictness or laxity of the selection; the result of the selection; whether it be the survival of the fittest, or only the elimination of the least fit; the constancy or occasional occurrence of Natural Selection; and the severity or mildness which characterises the struggle for existence.

The materials on which Natural Selection works are the variations which occur necessarily and inevitably in connection with the phenomenon of sexual reproduction; and obviously the nature attributed to them will largely influence the a priori credibility of the theory. If the variations are very slight, if they diverge in all directions, and if, therefore, only a few favourable variations occur, it is obvious that a stricter selection will be required than if the variations are considerable in quantity and if many favourable variations occur.

Mr. Darwin says that variations are slight. And yet he says that monstrosities graduate so insensibly into

mere variations [in which mere individual differences are included] that it is impossible to separate them.

Mr. Wallace says that variation is generally very small in amount; that variation is merely the absence of identity and yet he also says few persons consider how largely and universally all animals are varying.

A similar difference of opinion is found as to whether the same variation will occur in one or a few, or in many individuals.

Mr. Darwin says: "These individual differences afford materials for Natural Selection to act on."-(Origin of Species. p. 34.)

Professor Huxley says: "The variations from their specific type which individuals present are the objects of' the selective action of external conditions."

"The theory of Natural Selection, as such, furnishes no warrant for supposing that the same beneficial variety should arise in a number of individuals spontaneously. On the contrary, the theory of Natural Selection trusts to the chapter of accidents in the matter of variation; and in this chapter we read of no reasons why the same beneficial variation should arise simultaneously in a sufficient number of individual cases to prevent its being swamped by intercrossing with the parent species."

Hence it is contended that

"A very large assumption is made, when it is said that the same variation occurs simultaneously in a number of individuals inhabiting the same area."—(The Journal of the Linnean Society. vol. xix., Zoology, p. 343.)

To this, Mr. Wallace, speaking from actual observation of nature, makes the following reply:

"But that which Mr. Romanes regards as 'a very large assumption,' is, I maintain, a very general fact, and at the present time, one of the best established facts in natural history. A brief summary of these facts is given in my 'Island Life' (p. 57), and I possess in manuscript a considerable collection of additional facts, showing that simultaneous

variation is a general phenomenon among the best known species of animals and plants. Unfortunately very few naturalists pay attention to individual variations.”—(Fortnightly Review. vol. xl., p. 307.)

If we represent the differences which occur among the inevitable variations associated with sexual reproduction from the point of view of direction, we shall find a similar difference of opinion.

They are represented as divergent, as varying in all directions.

"Natural Selection, or survival of the fittest, among divergent varieties of offspring, is the distinctive Darwinian factor."-(Le Conte. Evolution and its Relation to Religious Thought. p. 74.)

"The theory requires-variations of all parts in all directions.— (Mivart. The Genesis of Species. p. 60.)

"All that this theory has to do, is to take the principle of promiscuous variation in all directions, as a datum supplied by observation." -(Romanes. Nature. vol. xxxiv., p. 360, note.)

"The Darwinist can point to the results of 'artificial selection' unquestionably to demonstrate that, in our domesticated plants, and animals, variation is so far omniferous as to lend itself to all the morphological changes in divergent lines which have been produced by the continuous, or cumulative, selection of the horticulturist and the breeder." (Romanes. Contemporary Review. vol. liii., p. 850.)

In the second place it is asserted that the variations cluster around a central line, from which a few variants diverge on either side.

Professor George H. Darwin says:—

"One may assume, with some confidence, that under normal conditions, the variation of any organ in the same species-may be symmetrically grouped about a centre of greatest density.”(Nature. vol. viii., p. 505.).

"If one were to draw a vertical line on a wall, and were to measure the heights of several thousand men of the same race against this line, recording the height of each by driving in a pin, the pins would be densely clustered about a certain height, and the density of their distribution would diminish above and below. Quetelet experimentally verified that the density of the pins at any distance

above the centre of the cluster was equal to that at a little distance below; he also found that the law of diminution of density on receding from the cluster was given by a certain mathematical expression, to which, however, I need here make no further reference. A similar law obtains with reference to the circumference of the chest; and one may assume, with some confidence, that, under normal conditions, the variation of any organ in the same species may be symmetrically grouped about a centre of greatest density, as above explained."-(Nature. vol. viii., p. 505.)

Mr. Wallace enunciates the same principle in the following passage:

"If now we consider the population of a species with regard to any particular character or combination of characters, we may divide it into three groups—a central group in which the mean or average development prevails with little variation, one in which the character is excessively, and one in which it is little developed. These groups would not be of equal extent, the central portion—that in which the mean characteristics prevailed-being in accordance with the law of averages, much more numerous than the extremes; perhaps twice or even three times as great as either of them, and forming such a series as the following:-Maximum development 10, mean 30, minimum 10. These figures, whatever their exact proportions, would probably be pretty constant, for we have no reason to believe that the mean characters, or the amount of variation of a species, change materially from year to year or from century to century; and we may therefore look upon the central and most numerous group as presenting the typical form of the species, being that which is best adapted to the conditions in which it has actually to exist, while the extremes, being less perfectly adapted, are continually weeded out by natural selection." (Nineteenth Century. vol. vii., pp. 100-1.)

And yet Mr. Wallace elsewhere arrives at a different result.

He says :—

"Mr. Allen also gives full details as to the variation of colour and marking, showing that these are not less striking than those of size and proportions; but the most important thing for us in regard to the question we are discussing is the amount of simultaneous variation of the same kind that is constantly occurring. To determine this I formed diagrams, in which each individual was represented by a spot placed on a horizontal line at a point determined by its actual dimensions. It would have been antecedently expected that the great bulk of the spots would be crowded together about a point

representing the mean dimensions of the species, but this was by no means the case. Often the central point was not all crowded with dots, but they were grouped with rough uniformity for a considerable distance on each side of the centre, with a few isolated at greater distances representing the extremes of variation. Hence a species could usually be divided into two portions, with a considerable number of specimens in each showing divergence from the mean condition--the very simultaneous variation' which Mr. Romanes regards as a very large assumption.'"-(The Fortnightly Review, vol. xl., N.S., p. 309.)

Variations are thus represented as divergent, i.e., radiating in all directions; as forming a cluster around a central line; and as forming two clusters at some little distance from the central line.

We find a similar discrepancy in the statements as to the strictness or the laxity of the selection which takes. place in nature.

Mr. Wallace says:—

"Selection is constantly

eliminating all that fall below

the best working standard, and preserving only those that are fully up to it.”—(Darwinism. p. 413.)

But elsewhere he speaks of—

"A struggle for existence, in which the weakest and least perfectly organised must always succumb."--(Contributions to the theory of Natural Selection. p. 33.)

He also says:

"Nature does not so much select special varieties as exterminate the most unfavourable ones."-(Wallace's Letter in Darwin's Life. iii., p. 46, note.)

Mr. Darwin says:

"We must suppose each new state of the instrument (the human eye) to be multiplied by the million each to be preserved until a better one is produced, and then the old ones to be all destroyed." --(Origin of Species. p. 146.)

On the other hand, the principle of a lax selection is asserted in the following passages by Mr. Darwin.