reference may be added the following passage written nearly fifty years ago to Sir Joseph Hooker:

'Two great classes of facts make me think that all variability is due to change in the conditions of life: firstly, that there i more variability and more monstrosities (and these graduat into each other) under unnatural domestic conditions tha under nature; and, secondly, that changed conditions affec in an especial manner the reproductive organs.'

De Vries himself admits that the plants now growing in the Nile Valley are precisely the same as those represented in the tombs and on the monuments of the earlier dynasties. Now, according to Sir W. Thiselton-Dyer, plants such as these, if removed from the uniform conditions under which they have grown for unknown thousands of years in this most fixed and stable portion of the earth's surface, and cultivated for a few generations in new and strange surroundings of climate, soil, manure, etc., will at once exhibit the most marked variation in many directions.

It is indeed probable that the plant, fixed to the spot where the seed germinates, has been given by natural selection a susceptible constitution which is influenced in the direction of variation by new conditions. Thus the adverse circumstances of the fixed mode of life may be compensated; for a plant may be stimulated by a strange environment to produce a highly variable offspring, thus greatly increasing the chance of successfully meeting that environment with an advantageous variation.

Natural Selection Creative.

De Vries concludes his volume of Californian lectures with the following quotation from Arthur Harris: 'Natural selection may explain the survival of the fittest, but it cannot explain the arrival of the fittest.' It is not surprising to find that the above-stated conclusion is accompanied by a very inadequate appreciation of the relative importance of the struggle for existence with the organic environment. Darwin, after long reflection on the conditions of existence, felt 'inclined to swear at the North Pole and . . . to speak disrespectfully of the

* March 18, 1862. More Letters,' i, 198.

Equator, because the struggle with living foes seemed
to him of so much greater importance than soil, climate,
etc. De Vries, as we see in the following sentence (p. 120),
returns to the pre-Darwinian order of importance: 'Of
course, nert to the climate and soil in importance, come
ecological aditions, the vegetable and animal enemies
of the plants, and other influences of the same nature.'

The criticism that natural selection is not creative is
by no means new. It has been urged from time to time
by arious writers ever since the first appearance of the
Origin of Species. One of the most striking forms of
the criticism is to be found in the following passage in
an article by the Duke of Argyll :

Strictly speaking, therefore, Mr Darwin's theory is not a theory on the Origin of Species at all, but only a theory on the causes which lead to the relative success and failure of such new forms as may be born into the world.' †

In this case we have the advantage of knowing precisely what Darwin thought of the criticism, for he wrote to Sir Charles Lyell :

'I demur also to the Duke's expression of "new births." That may be a very good theory, but it is not mine, unless indeed he calls a bird born with a beak one hundredth of an inch longer than usual "a new birth"; but this is not the sense in which the term would usually be understood. The more I work, the more I feel convinced that it is by the accumulation of such extremely slight variations that new that I forget that natural selection means only the preservaspecies arise. I do not plead guilty to the Duke's charge, tion of variations which independently arise.

I have ex

pressed this in as strong language as I could use, but it would
have been infinitely tedious had I on every occasion thus
garded myself. I will cry "peccavi" when I hear of the
Dake or you attacking breeders for saying that man has
made his improved shorthorns, or pouter pigeons, or bantams.
And I could quote still stronger expressions used by agricul-
turists, Man does make his artificial breeds, for his selective
power is of much importance relatively to that of the slight
spontaneous variations. But no one will attack breeders for

'To Sir Charles Lyell, October 11, 1859. 'Life and Letters,' ii, 212.
Scotsman,' December 6, 1864, Quoted in 'Life and Letters of Charles

Darwin,' iii, 33, note.

using such expressions, and the rising generation will no

blame me.'

Further, it must be remembered that Darwin believe that these latter variations selected by man were much larger than the minute differences selected by nature. I this conclusion be admitted, it follows that there is far more justification for saying that Nature 'makes' her species than that man 'makes' his artificial breeds.

The creative power of natural selection, due to the accumulation and combination by it of small useful variations, is also clearly set forth in a letter written by Darwin to Asa Gray a few days after the publication of the 'Origin' on November 24, 1859:

'I had not thought of your objection of my using the term “natural selection" as an agent. I use it much as a geologist does the word denudation-for an agent, expressing the result of several combined actions. I will take care to explain, not merely by inference, what I mean by the term; for I must use it, otherwise I should incessantly have to expand it into some such (here miserably expressed) formula as the following: "The tendency to the preservation (owing to the severe struggle for life to which all organic beings at some time or generation are exposed) of any, the slightest, variation in any part, which is of the slightest use or favourable to the life of the individual which has thus varied; together with the tendency to its inheritance." Any variation, which was of nó use whatever to the individual, would not be preserved by this process of "natural selection.”' †

Natural selection, as above set forth by Darwin himself, has a great deal to do with the arrival' or building up of the fittest.

We have considered de Vries' hypothesis of evolution by mutation at some length, because its establishment would mean a profound change in the ideas received from Darwin. The survival of the fittest among a crowd of fresh elementary species or sub-species offered readymade by mutation is a very different conception from that of the progressive building-up of the fittest types by the improvement through selection of existing characters

* Jan. 22, 1865. Life and Letters,' iii, 33.

† November 29, 1859. More Letters,' i, 126, 127,

and qualities, and the gradual addition of a unit here and a unit there to the complex structure of a species.

Mendel's Law.

On the other hand, the epoch-making researches of Mendel do not greatly affect the Darwinian position. They easile us to trace the history and fate of the germs of certain single characters, just as the microscope has enabled us to trace the history and fate of the germ of the hole individual. Characters which follow Mendel's lar behave as if they were separate units each represented in the germ-cell by its own minute germ or determinant. When the germ-nuclei unite in fertilisation, the representatives of characters also unite; so that each unit

character in the offspring developes from a double determinant, of which half was contributed by the germ-cell of one parent and half by the germ-cell of the other. This double composition of every individual animal or plant produced from a fertilised germ-cell is well emphasised by Prof. Bateson in his Inaugural Lecture:

'Simple though the fact is, I have noticed that to many it is difficult to assimilate as a working idea. We are accustomed to think of a man, a butterfly, or an apple tree as each one we must get thoroughly familiar with the fact that they are thing. In order to understand the significance of Mendelism, each two things, double throughout every part of their com

position' (p. 6).

Mendel's law was established by breeding together individuals in which a character appeared in two alternate forms, or in which a character in one parent was represented by a different character in the other. The double character in the next generation was thus compelled by its origin to possess a cross-bred constitution, composed, It must be remembered that such a Mendelian (or allelomorphic) pair may be made up of a character derived from one parent and the want of it derived from the other. In fact Prof. Bateson, in his latest work,

ait inevitably was, of a pair of dissimilar units.

maintains that,

...as knowledge of Mendelian cases has increased, the Applicability of what is here spoken of as the "presence and absence" hypothesis becomes more and more clear' (p. 286).

The double composition of every character, due to its double parentage, has been insisted on. From this con-sideration it follows that the parental germ-cells which unite in fertilisation contain half and not whole, or single and not double, determinants of characters. If the determinants were not thus halved, they would after fertilisation be doubled in size and would be doubled again in each succeeding generation. As it is, the double determinants can maintain a uniform size. By tracing the history, in the succeeding inbred generations, of the alternative characters which he compelled (in the first generation) to pair, Mendel found that the determinants (or allelomorphs) were not merely halved in size but split into single determinants similar to those received from the two original parents. Such characters are said to obey Mendel's law.

'This phenomenon, the dissociation of characters from each other in the course of the formation of the germs, we speak of as segregation; and the characters which segregate from each other are described as allelomorphic, i.e. alternative to each other in the constitution of the gametes.' (Bateson, p. 11.)

The fact of segregation-according to Prof. Bateson (p. 13), the essential discovery made by Mendel-leads naturally to the broader inference that all characters are represented by parental halves, but that in non-Mendelian characters their union is so complete that the splitting for the germ-cells of the next generation does not follow the line of original fusion. Of course, the expression 'line of fusion' is only used metaphorically; what is not metaphorical is the conception that each half is, in the first case, withdrawn pure from the other, but that, in the second, mutual interpenetration is so complete that such withdrawal is no longer possible. We thus arrive at a conception of double determinants and a double composition for every character of the organism, whether subject or not subject to the Mendelian principle.

A full account of the details of Mendel's experiments will be found in the works of Prof. A. Thomson and Mr R. H. Lock; while Prof. Bateson's recently published volume contains a valuable summary of the researches conducted on Mendelian lines up to the present date, and includes a translation of Mendel's original memoir,