That cach parent contributes almost equally to the offspring suggests the two-sided responsibility of parentage. As regards Mendelian characters the offspring will show with more or less completeness the dominant characters of its parents; as regards blending characters the inheritance is probably in the first instance dual, but beyond that multiple, for ancestral contributions may go to build up the final mosaic result.

If we adopt Weismann's conclusion that individually acquired modifications are not transmitted, we are saved from the pessimism suggested by the abnormal functions and environments of our civilisation.

And just in proportion as we doubt the transmission of desirable acquired characters, so much the more should we desire to secure that improved conditions of life foster the individual development of each successive generation.

That pathological conditions, innate in the organism, tend to be transmitted, suggests that men should be informed and educated as to the undesirability of parentage on the part of abnormal members of the community.

The widest problems of heredity are raised when we substitute "fraternities" for individuals, or make the transition to social inheritance—the relation between the successive generations of a society.

Thus Sir Francis Galton called attention to the regularity observed in the peculiarities of great populations throughout a series of generations. "The large do not always beget the large, nor the small the small; but yet the observed proportion between the large and the small, in each degree of size and in every quality, hardly varies from one generation to another." A specific average is sustained. This is not because each individual leaves his like behind him, for this is not the case. It is rather due to the fact of a regular regression or deviation which brings the offspring of extraordinary parents in a definite ratio nearer the average of the stock.

"However paradoxical it may appear at first sight, it is theoretically a necessary fact, and one that is clearly confirmed by observation, that the stature of the adult

offspring must on the whole be the more mediocre than the stature of their parents-that is to say, more near to the median stature of the general population." "Each peculiarity in a man is shared by his kinsmen, but on the average in a less degree. It is reduced to a definite fraction of its amount, quite independently of what its amount might be. The fraction differs in different orders of kinship, becoming smaller as they are more remote.' Yet it must not be supposed that the value of a good stock is under-estimated by Galton, for he shows how the offspring of two ordinary members of a gifted stock will not regress like the offspring of a couple equal in gifts to the former, but belonging to a poorer stock, above the average of which they have risen.

Yet the fact of regression tells against the full transmission of any signal talent. Children are not likely to differ from mediocrity so widely as their parents. "The more bountifully the parent is gifted by nature, the more rare will be his good fortune if he begets a son who is as richly endowed as himself, and still more so if he has a son who is endowed more largely." But "The law is even-handed; it levies an equal succession-tax on the transmission of badness as of goodness. If it discourages the extravagant hope of a gifted parent that his children will inherit all his powers, it no less discountenances extravagant fears that they will inherit all his weakness and disease."

The study of individual inheritance, as in Galton's Hereditary Genius, may tend to develop an aristocratic. and justifiable pride of race when a gifted lineage is verifiable for generations. It may lead to despair if the records of family diseases be subjected to investigation.

But the study of social inheritance is at once more democratic and less pessimistic. The nation is a vast fraternity, with an average towards which the noble tend, but to which the offspring of the under-average as surely approximate.

The importance of this consideration may be more readily appreciated if we take an actual case from Professor Karl Pearson's Grammar of Science.

"Fathers of a given height have not sons all of a given height, but an array of sons of a mean height different from that of the father and nearer to the mean height of sons in general. Thus take fathers of stature 72 in., the mean height of their sons is 708 in., or we have a regression towards the mean of the general population. On the other hand, fathers with a mean height of 66 in. give a group of sons of mean height 68.3 in., or they have progressed towards the mean of the general population of sons. The father with a great excess of the character contributes sons with an excess, but a less excess of it; the father with a great defect of the character contributes sons with a defect, but less defect of it. The general result is a sensible stability of type and variation from generation to generation.'

It must be understood, however, that these statistical generalisations are average statements for fraternities, not physiological conclusions relating to individuals. They do not apply to lineages where there has been close and consistent selection. They do not apply to the inheritance of Mendelian "unit characters." Nor do they take sufficient account of the important fact that resemblances are in part due to similar nurture, not wholly to inheritance of similar inborn characters.

It may be useful to keep the term "social heritage for what is supremely important in mankind, the registration of experience and achievements which is effected by tradition and custom, by institutions and laws, by literature and art. For these bulk largely in the determination of human life.

CHAPTER XX

THE INFLUENCE OF FUNCTION AND ENVIRONMENT

1. The influence of function-2. The influence of surroundings -3. The transmissibility of acquired characters or somatic modifications 4. The importance of nurture-5. The other side of heredity.

1. The Influence of Function.—A skilled observer can often discern a man's occupation from his physiognomy, his shoulders, or his hands. In some unhealthy occupations the death-rate is three times that in others. Disuse of such organs as muscles tends to their degeneration, while increased exercise is within certain limits associated with increased development.

Precise illustrations of the influence of function on animals are far from being abundant. Three sample cases may be cited. (1) A Japanese investigator, Shinkishi Hatai, has shown in the case of the white rat that long-continued exercise markedly increases the weight of the heart, kidneys, and liver, on an average to about 20 per cent. He exercised the rats for 90-180 days, which is comparable to a period of 7-14 years in man, for the span of life in the white rat is about three years. (2) Semper and De Varigny found that when freshwater snails were reared in vessels of a shape that allowed them abundant water but very little surface on which to take exercise they developed into dwarf forms. Every precaution was taken to secure abundant food, perfect aeration, and thorough removal of waste-products. De Varigny's experiments were particularly careful and point convincingly to the conclusion that the con

dition of dwarfing was the restricted area for exercise. (3) The results of physical exercises show that the size and strength of a muscle may be greatly increased by persistent practice. This is well known in the legs of professional dancers, in the powerful wrist of the violinist, and in the arms of the blacksmith. A force de forger on devient forgeron.

Even if we could gather many illustrations of the influence of use and disuse on individual animals, we should still have to find out whether the peculiarities or modifications acquired by individuals were in any representative way transmissible to the offspring, or whether any secondary effects of the acquired characters were transmissible, or whether these changes had no effect upon succeeding generations.

It is easy to find hundreds of cases in which the constant characters of animals may be hypothetically interpreted as the result of use or disuse. Is the torpedo-like shape of swift swimmers due to their rapid motion through the water, do burrowing animals necessarily become wormlike, has the giraffe lengthened its neck by stretching it, have hoofs been developed by running on hard ground, åre horns responses to butting, are diverse shapes of teeth the results of chewing diverse kinds of food, are cave-animals blind because they have ceased to use their eyes, do the wing bones and muscles of the domesticated duck compare unfavourably with those of the wild duck because the habit of sustained flight has been lost by the former ?

To questions of this sort it must be answered that the facts at present known do not afford much support to the theory of the direct origin of adaptations." It is infinitely easy," Semper says, "to form a fanciful idea as to how this or that may be hypothetically explained, and very little trouble is needed to imagine some process by which hypothetical fundamental causes-equally fanciful-may have led to the result which has been actually observed. But when we try to prove by experiment that this imaginary process of development is indeed the true and inevitable one, much time and