the

Granted that the progressive modification of the limb-
bones of the horse caused progressive improvement in
power of erading enemies and obtaining food, how
can we expect the fossils of any particular horizon to
demonstrates higher rate of extinction in the individuals
which were below the average in this respect? All we
can expect to find, as regards any character that exhibits
progresie improvement, is evidence consistent with the
belief that the individuals of each horizon were, upon the
whole, descended from the more highly endowed and not
f the less highly endowed individuals of the horizon
immediately preceding it in order of time. And this is
precisely what we do find when the record is complete.
Any larger expectation appears to be based on a miscon-
ception of the criteria which lead to success or failure
in the struggle for existence. A crude and extreme form
of this misconception, published in the North American
Review' for April 1860, was referred to by Darwin in a
letter to Sir Joseph Hooker:*

Asa Gray has sent me an article from the United States,
clever, and dead against me.
But one argument is funny.
The reviewer says, that if the doctrine were true, geological
strata would be full of monsters which have failed. A very
clear view this writer had of the struggle for existence!'

We

may set beside the wonderful series of American vertebrate fossils the evidence afforded by the study of invertebrate fossils from the zones of a rich European formation. Such records preserved in the white chalk

of this country have been investigated with extraordinary

Dess

minuteness and accuracy by Dr Arthur W. Rowe, who has
kindly prepared for me the following brief account of
the general conclusions to which his labours have led.
The white chalk of England offers an almost unique
field for observations of the kind, on account of its thick-
(considerably over 1000 ft.), its slow, uniform, and
continuous deposition in a sea of moderate depth, with
to closely adjacent land, the abundance and the wonder-
tal state of preservation of its fossils, together with the
ality with which they can be cleared of the chalky
Apr 18, 1860. By Professor Bowen' is written on Darwin's copy of the
Life and Letters of Charles Darwin,' ii, 304. The letter is dated

matrix. Among the commonest of chalk fossils are flattened heart-shaped sea-urchins of the genus Micraster These are first found as thin-walled, sparsely ornamented forms from which spring, as we ascend the zones, all the other species of the genus. The progression is unbroken and minute in the last degree. We can connect together into a continuous series each minute variation and each species by gradations of structure so insensible that not a link in the chain of evidence is wanting. In the other common sea-urchins of the chalk, Echinocorys and Conulus, although evidence derived from the details of structure is not equally available, that afforded by the gradual variations in shape as we ascend through the zones of the formation is convincing and complete. Equally clear proof of continuous evolution is provided by the study of the belemnite Actinocamax. Although this genus reaches at definite zonal levels a sufficiently accentuated degree of variation in its intrinsic characters as to warrant, for purely stratigraphical purposes, a trivial title, the fact remains that these so-called species are but landmarks in the progressive and unbroken evolution of a single though somewhat plastic genus.' The bearing of this evidence upon the question of continuity or discontinuity in evolution is of paramount importance. Nowhere has the evidence been collected so fully as in the case of the white chalk; nowhere have such conclusive proofs of continuity in evolution been established.

The bearing of the fossil record upon this question was from the first appreciated by Darwin, who, in giving to Asa Gray an account of the arguments by which he had been led to maintain the gradual as against the sudden evolution of species, points to the following evidence: Look at the fineness of gradation in the shells of successive sub-stages of the same great formation.'t In a letter to the Marquis de Saporta, he wrote: 'Nothing can be more important, in my opinion, than your evidence of the extremely slow and gradual manner in which specific forms change.'‡

* Proc. Geol. Soc.,' vol. xviii, pt. 4 (1904), pp. 274, 275.

The

To Asa Gray, August 11, 1860. ('Life and Letters,' ii, 334.) whole passage, which is of great importance, is quoted above on pp. 11, 12. Referring to Saporta's Études sur la Végétation,' etc., May 30, 1874. ('Life and Letters,' iii, 188.)

In this as in many other directions the laborious and detailed studies of recent years only serve to confirm the conclusions which the genius of Darwin inferred from the starty data available half a century ago.

Evolution by Mutation is not evident where Natural
Selection is least active.

The opinion has been sometimes expressed that the
new mutations which arise spontaneously have no chance
of establishing themselves, because of the repressive power
of natural selection; and that, if this power were in
beyance, an evolution by mutation would ensue. Thus
Mr R. H. Lock, speaking of plants, has argued as follows:
that natural conditions lead to the obliteration of a host of
mutations, is as fair a deduction from the fact that such
mutations appear under cultivation as the current deduction
that the conditions of cultivation actually cause the occurrence
of this kind of variation. (Nature,' lxxvi, 616.)

But, as Darwin recognised long ago, it is precisely where natural selection is most actively at work that rapid evolution occurs, and where natural selection is least operative that ancestral forms persist with little change.

For instance, as regards the Platanistidæ or freshwater porpoises of the Indus, Ganges, and Amazons, which Occupy an intermediate, and probably ancestral, position among the toothed whales, Darwin wrote to Sir J. Hooker: 'I cannot doubt we here have a good instance, precisely like that of ganoid fishes, of a large ancient marine group, preserved exclusively in fresh-water, where there has been competition, and consequently little modification.' *

When we

study the records of life and its changes

preserved for us in the archives of palæontology and geographical distribution, we fail to find any evidence of an evolution urged from within the organism by the periodic mutations suggested to de Vries by his experience with a single problematic form-Enothera lamarcki

Wherever, as on the floor of the deep ocean, con

tions have been stereotyped for the longest periods of me, precisely there have the organisms themselves been Ingest stereotyped. Yet the relatively simple conditions

of such scantily-peopled areas are just those which would admit of an evolution by mutation without selection, it such evolution were ever possible.

Not only do we find long persistence of organisms ir the less crowded areas, but we also encounter the long persistence of organs, or of the arrangement of organs which are farthest withdrawn from the active life of the possessor, and are therefore the least likely to be acted on by natural selection. For this reason such structures are especially valuable in classification. The modifica tion and growth of Darwin's opinion on this subject is shown in two of his letters to Sir Joseph Hooker:

'Whilst looking for notes on the variability of the divisions of the ovarium, position of the ovules, æstivation, etc., I found remarks written fifteen or twenty years ago, showing that I then supposed that characters which were nearly uniform throughout whole groups must be of high vital importance to the plants themselves; consequently I was greatly puzzled how, with organisms having very different habits of life, this uniformity could have been acquired through Natural Selection. Now, I am much inclined to believe, in accordance with the view given towards the close of my MS., that the near approach to uniformity in such structures depends on their not being of vital importance, and therefore not being acted on by Natural Selection.'*

It is evident that Sir Joseph agreed with the later interpretation; and Darwin replied as follows:

'It is very true what you say about unimportant characters being so important systematically; yet it is hardly paradoxical, bearing in mind that the natural system is genetic, and that we have to discover the genealogies anyhow. Hence such parts as organs of generation are so useful for classification though not concerned with the manner of life. Hence use for same purpose of rudimentary organs, etc. You cannot think what a relief it is that you do not object to this view, for it removes partly a heavy burden from my shoulders.'†

If mutations at all freely established themselves in those parts of the organism where the structural arrangements are the heritage of a long past, and where we have

no reason to suppose that natural selection would intervene, the basis of classification and the determinations of homology and affinity would not be nearly as secure as we now find them to be.

The Causes of the great Variability of Cultivated

Plants.

The great variability of cultivated species of plants is ased by de Vries (pp. 65, 66) to be due to an original maliformity. He holds that the more variable species Tere often deliberately chosen by man because of their prester promise. Further, speaking of the improvement of cultivated plants both older and newer, he says (p. 92): In either case the starting-point is as important as the improvement; or rather the results depend in a far higher degree on the adequate choice of the initial material than on the methodical and careful treatment of the chosen varieties.' These conclusions are entirely the reverse of those arrived at by Darwin, Asa Gray, Sir Joseph Hooker, and Sir W. Thiselton-Dyer. The last-named authority, in an important letter entitled 'Specific Stability and Mutation,' comes to the following conclusions, for which he brings forward much evidence:

'While specific stability under constant conditions appears to be the rule in nature, it is widely different in cultivation. When a plant is brought under cultural conditions it maintains its type for some time unaltered, then gives way and

becomes practically plastic' (p. 78).

Not only do the above-quoted words sum up the long be I have been informed by him that they equally experience of the author as Director of the Kew Gardens, express the opinion of his illustrious predecessor, Sir Joseph Hooker, and the conclusions of the existing staff. In the letter itself, Sir W. Thiselton-Dyer quotes the Concordant views of Asa Gray, in 'Darwiniana' (pp. 338347), and of Charles Darwin, in Animals and Plants der Domestication' (vol. ii, p. 261). To the latter

the conclusions published by R. H. Lock in 'Nature,' lxxvi, 616. Mr Lock's Nature,' (November 28, 1907), 1xxvii, 77-79. The letter is a reply to a contention is quoted on p. 21 of the present article.

i