idea of an internal force, compelling development in certain directions, and especially towards higher organisation, which was common eleven years ago, is still held by some naturalists; but it has almost vanished before the accumulating testimony of palæontology, that several of the lower organisms have existed from the earliest part of the Palæozoic era up to the present day, without undergoing any important change in their hard parts which alone have been preserved—and therefore probably without much change in their soft parts either.

Indeed, most of the classes of animals and plants have undergone but little change since their first appearance. In comparatively few cases has change been rapid; but it is these rapidly changing forms which seem so remarkable in our eyes, and give the impression that great change is more universal than it really is. Except the large Lycopods and Crustaceans of the newer Palæozoic, the reptiles and birds of the Mesozoic, and perhaps a few of the Eocene hoofed mammals, there is nothing among extinct plants and animals that would appeal to the untrained eye as anything remarkable and unlike living plants and animals. Our views on this subject are much exaggerated, owing to the numerous drawings and models that have been made of a few of the most extraordinary of the animals; and we forget that they were only a few among a host of quite ordinary beings.

DEFINITE AND INDEFINITE VARIATION

In the second lecture I have entered somewhat fully into the difference between Lamarckism and Darwinism, because I have noticed misconceptions on

the subject among several writers on evolution; and it was necessary to make the difference as clear as possible, without going into technical details. There seems to be especial confusion on the subject of definite or determinate, and indefinite or indeterminate variation; and a few more words about it may not be out of place in this Introduction; for, although not included in Darwinism, it has an indirect bearing on our views of natural selection.

Some writers seem to think that because determinate variation implies a directing agency, therefore indeterminate variation can have no directing agency, and that it has no cause at all. This confusion is largely due to the unfortunate substitution of the terms "determinate" and "indeterminate" for Darwin's "definite" and "indefinite" variation, which are far more appropriate. I have also seen it stated that, if variations are definite, there is nothing left for natural selection to do; which is quite a mistake. A definite variation may be injurious, or it may be indifferent, or it may be useful to its possessor. The first will be checked, the second will be left alone, and the third will be still further developed by natural selection. The mistake has arisen through supposing that all definite variations must be useful, which would no doubt be the case if all definite variations were due to use-inheritance; but we have no reason to suppose that this is so. This is clearly seen when we consider mental variations, many of which are definite, that is, are impressed simultaneously on a large number of individuals, but are subject to the laws of selection, as is pointed out in the third lecture.

"Definite variation" only means that a variation,

gradually increasing in intensity, is transmitted by a number of individuals from one generation to the next. While "indefinite variation" means that each variation is individual only, and may or may not die out with the individual. Individual variations are often transmitted from one generation to another; and the oftener they are transmitted, the more constant they become; until, at last, a large number of individuals constantly acquire the same character. Here we have cases of indefinite changing into definite variation, showing that the two are fundamentally the same; the difference between them lying in the strength and constancy of the directing cause.

Variations must be due either, (1) to the action of the environment, directly on the individuals, or indirectly, by causing a change of habit, and thus leading to the greater or less use of certain organs. Or, (2) to internal causes affecting the action of some law of growth which counteracts the law of heredity. Each of these causes may possibly give rise to indefinite or to definite variation, according to the strength with which it acts. We have no reason to suppose that external causes must act identically on different individuals, or that because one individual changes its habits therefore many must do the same; but no doubt the environment, if it acts at all, would generally affect a large number of individuals simultaneously, and post-natal variations would generally be definite. On the other hand, indefinite variations would generally arise during the development of the individual; and consequently they must generally be congenital in origin, although the effects may not show until long after birth.

Definite variation is due to the directive force being sufficiently strong to overcome all obstacles. The greatest obstacle is free intercrossing with individuals which do not possess the variation. This is overcome either by the isolation of a few individuals, or by a large number of individuals changing together in the same direction, and so forming a group by themselves. This last is the necessary foundation of Lamarckism; and the term "determinate variation" might be restricted to it, provided it was well understood that determinate variation was only a special form of definite variation. The first is the foundation of the new Darwinism; but that theory does not exclude definite variation, or even that particular form of it which we have just called determinate variation, if it should hereafter be shown that such variations can be transmitted from one generation to another. Indeed, it is well known that Mr Darwin allowed far more influence to use-inheritance than the new Darwinians are inclined to do.

THE TRANSMISSION OF ACQUIRED CHARACTERS

That post-natally acquired mental characters or variations can be transmitted, we know from many undoubted cases of inherited habits; but it does not necessarily follow that post-natally acquired structural characters can be transmitted. For mental variations are transmitted in two ways: first by imitation or education, and secondly by inheritance. It is pro

bable that the second form can only occur after the variation has been transmitted by imitation through several generations, during which it passes from the indefinite to the definite stage.

This cannot take place with structural variations ; and a post-natally acquired character has but a small chance of becoming a congenital character, unless it is reproduced generation after generation by the action of climate or other external agent. It might then perhaps in time become definite and congenital, provided there is some process by which structural variations affect the germ-cells in a similar manner to the action of mental variations on the brain-cells, which— although we cannot explain it we know to be a fact by the phenomenon of memory. And, as something very similar to memory appears to be the cause of structural development, this may be the case; the evidence bearing on the subject, however, is weak and contradictory. Mr Darwin offered an explanation in his hypothesis of Pangenesis; but, for reasons given in the first lecture, that hypothesis cannot be maintained, at any rate in its integrity.

"1

It is thought by some that "if natural selection be inadequate to explain many of the facts of evolution, there is no alternative but the view that development is partly caused by the transmission of changes brought about in the organism as a result of its own activity, directed and conditioned by the environment, and of the action of external agencies." Which, I suppose, means that there is no alternative but a partial return to Lamarckism. It will, however, be seen in the second lecture that there is no necessity for thinking that acquired characters must be inherited in order to supplement the action of natural selection. The isolation of variations, no matter how they have arisen, is sufficient to explain all the deficiencies without any 1 Parker and Haswell, "Text-book of Zoology," vol. ii., p. 627.