direct line of our ancestry. To the Neanderthal line may be referred the Heidelberg man of the early Pleistocene. A second collateral line which has come to nothing is that represented by the Piltdown skull (Eoanthropus dawsoni) of the end of the Pliocene. A third collateral line, perhaps diverging in the Miocene, is represented by Pithecanthropus erectus at the end of the Pliocene. According to Keith, these three lines are separate offshootsall eventually failures-diverging from the humanoid stem. As it is certain that man could not have arisen from any of the known anthropoid apes, and as it is likely that he arose from a stock common to them and to him, it seems justifiable to date the antiquity of the human race not later than the time when the anthropoid apes are known to have been established as a distinct family. This takes us back to pre-Miocene ages. If we mean by the antiquity of man the period since the human stem differentiated from that which led on to the great anthropoid apes, it may be estimated (Keith and Sollas)1 at about two million years. If we mean the period at which the brain of man reached a human level or standard, we have," says Keith, "reasonable grounds for presuming that man had reached the human standard by the commencement of the Pliocene period,”—about a million years ago. Amid much uncertainty the general result of recent investigation is clear, that the modern type of man is vastly older than was supposed even in the days of Lyell's Antiquity of Man (1863).

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Converging facts point to the conclusion that man arose as a mutation (a discontinuous variation. of considerable magnitude) in a stock common to him and to the anthropoid apes. In regard to the factors which helped to secure man's ascent we can only speculate."

(a) From what we know of men and monkeys, it seems likely that in the struggles of primitive man cunning was more important than strength, and if intelligence now became, more than ever before, the condition of life or death, wits would tend to develop rapidly.

1 Sollas, Ancient Hunters, 1911.

2 The author's Outlines of Zoology, 6th edition, 1914, p. 293.

(b) When habits of using sticks and stones, of building shelters, of living in families began and some monkeys exhibit these-it is likely that wits would increase by leaps and bounds.

(c) Professor Fiske and others have emphasised the importance of prolonged infancy, and this must surely have helped to evolve the gentleness of mankind.

(d) Among many monkeys society has begun. Families combine for protection, and the combination favours the development both of emotional and intellectual strength. Surely "man did not make society, society made man.'

CHAPTER XIX

HEREDITY

1. The facts of heredity-2. Theories of heredity-historical retrospect-3. The idea of the continuity of generations-4. Modes of inheritance-5. Social and ethical aspects.

IN olden times men spoke of the three Fates which were believed to determine of what sort a life should be. With the decay of poetic feeling, and in the light of common science, the forms of the three sisters have faded. But they are realities still, for men are thinking more and more vividly about the factors of life. The biological factors are Heredity, Function, and Environment: the capital with which a life begins, the interest accruing from the investment of this in varied vital activities, and the force of circumstancés. But while it is useful to think of Heredity, Function, and Environment as the three Fates, we must not mystify matters by talking as if these were entities acting upon the organism. They are simply aspects of the fact that the creature is born and lives. The inheritance includes all that the organism is or has to start with when it begins its life as a fertilised eggcell, and heredity is only a name for the genetic relation between successive generations. Moreover, the functions of an organism depend upon the inherited and the acquired constitution, and so does its susceptibility to the influences of environment. Function includes all the actions and reactions between the living creature and its surroundings, and its influence may operate through disuse as well as through exercise.

We define heredity as the relation of organic or genetic

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continuity between successive generations, choosing this definition because it is misleading to talk about "heredity" as a "basal principle in evolution," as a great law," as a power, or as a cause." When we call heredity a "Fate," it is plain that we speak fancifully, but principle" and "law" are dangerous words to play with. We cannot think of life without this organic relation between parents and offspring, and had species been created instead of having been evolved there would still be heredity.

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1. The Facts of Heredity. An animal sometimes arises as a bud from its parent, and in rare cases from an egg which requires no fertilisation, but apart from these exceptions every multicellular animal develops from an egg-cell with which a male-cell has united in an intimate way. The egg-cell supplies most of the initial living matter, but the nucleus of the fertilised egg-cell is formed in half from the nucleus of the immature ovum, in half from the nucleus of the spermatozoon. Each parent usually contributes the same amount of nuclear material -the same number of chromosomes to the offspring, and this nuclear stuff is very essential.

In some cases (few as yet known) half of the spermatozoa have the same number of chromosomes as the ovum, while the others have one less. There is clear evidence in some of these cases that an egg fertilised by a spermatozoon with the same number of chromosomes develops into a female, while an egg fertilised by the other type of spermatozoon (with one chromosome less) develops into a male. The extra chromosome, which half of the spermatozoa have and half have not, is called the x-element or accessory chromosome, and it seems as if the presence of two x-elements in the fertilised ovum liberated the quality of femaleness, while in the presence of a single x-element maleness finds expression.

Another fact is more obvious, the offspring is very like its kind. One of the first things that people say about an infant is that it is like its father or its mother or some near relative, and the assertion does not arouse any surprise, although the statement, often more obviously true,

that the infant is like any other of the same race is received with contempt. But every one admits that "like begets like."

This likeness between offspring and parent is often far more than a general resemblance, for peculiar features and minute idiosyncrasies are frequently reproduced. Yet one must not assume that because a child twirls his thumbs in the same way as his father did, the habit has been inherited. For peculiar habits may readily reappear by imitation, and peculiarities of structure

FIG. 116.-DEVONSHIRE PONY, SHOWING THE OCCASIONAL OCCURRENCE OF ANCESTRAL STRIPES.

because the offspring grow up in conditions similar to those in which the parents lived.

Abnormal as well as normal characters, innate in the parents, may reappear in their descendants, and the list of weaknesses and malformations which may be transmitted is long and grim. But care is required to distinguish between reappearance due to inheritance and reappearance due to similar conditions of life.

Then there is a strange series of facts showing that an organism may reproduce characteristics which the parents