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 pos

sess 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 sec

ondly by inheritance. It is probable 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 postnatally 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 manner similar 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.

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

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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 help from Lamarckism. Indeed, it is not clear how Lamarckism bears on the case at all; for that is an hypothesis to exI plain the origin of adaptations only; and fails, equally with natural selection, to explain nonutilitarian characters, including the mutual sterility between species. There may be special cases, in which the direct action of external agencies has been the prominent factor in the formation of a new species; but certainly the failure of natural selection to explain all the phenomena of evolution does not oblige us to believe in Lamarckism, either wholly or in part.

' Parker and Haswell, Text-Book of Zoology, vol. ii., p. 627.

PHYSIOLOGICAL SELECTION

I have not used Dr. Romanes's term "physiological selection," because, except in the Bacteria, species are not founded on physiological characters; and, therefore, no amount of physiological selection could give rise to a new species; it can at most only produce isolation of some individuals from others. The best case of physiological selection is what the Rev. G. Henslow has called Constitutional Selection," which is a struggle for existence in which the sick and weakly give way to the healthy and strong. His illustrations are taken from thickly growing seedlings of the same species of plant; and it is evident that this would not differentiate two morphological varieties; Dr. Romanes himself did not suppose that his physiological selection could directly originate species. He always described it as a form of isolation; as also does Professor Lloyd Morgan in his Animal Life and Intelligence. Under these circumstances I think it will avoid ambiguity if the "physiological selection" of Dr. Romanes is included as a form of physiological isolation; and I have called it "progressive infertility."

SOCIAL EVOLUTION

The principle of Selection was divided by Mr.