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ns of the high plateaus of Peru show a striking development of lungs and thorax； as a result of living constantly at high altitudes。
Such special forms of variation as arrests of development （microcephalism） and reversion to lower forms are next discussed。 Darwin himself felt （〃Descent of Man〃； page 54。） that these subjects are so nearly related to the cases mentioned in the first chapter； that many of them might as well have been dealt with there。 It seems to me that it would have been better so； for the citation of additional instances of reversion at this place rather disturbs the logical sequence of his ideas as to the conditions which have brought about the evolution of man from lower forms。 The instances of reversion here discussed are microcephalism； which Darwin wrongly interpreted as atavistic； supernumerary mammae； supernumerary digits； bicornuate uterus； the development of abnormal muscles； and so on。 Brief mention is also made of correlative variations observed in man。
Darwin next discusses the question as to the manner in which man attained to the erect position from the state of a climbing quadruped。 Here again he puts the influence of Natural Selection in the first rank。 The immediate progenitors of man had to maintain a struggle for existence in which success was to the more intelligent； and to those with social instincts。 The hand of these climbing ancestors； which had little skill and served mainly for locomotion； could only undergo further development when some early member of the Primate series came to live more on the ground and less among trees。
A bipedal existence thus became possible； and with it the liberation of the hand from locomotion； and the one…sided development of the human foot。 The upright position brought about correlated variations in the bodily structure； with the free use of the hand it became possible to manufacture weapons and to use them； and this again resulted in a degeneration of the powerful canine teeth and the jaws； which were then no longer necessary for defence。 Above all； however； the intelligence immediately increased； and with it skull and brain。 The nakedness of man； and the absence of a tail （rudimentariness of the tail vertebrae） are next discussed。 Darwin is inclined to attribute the nakedness of man； not to the action of natural selection on ancestors who originally inhabited a tropical land； but to sexual selection； which； for aesthetic reasons； brought about the loss of the hairy covering in man； or primarily in woman。 An interesting discussion of the loss of the tail； which； however； man shares with the anthropoid apes； some other monkeys and lemurs； forms the conclusion of the almost superabundant material which Darwin worked up in the second chapter。 His object was to show that some of the most distinctive human characters are in all probability directly or indirectly due to natural selection。 With characteristic modesty he adds （〃Descent of Man〃； page 92。）： 〃Hence； if I have erred in giving to natural selection great power； which I am very far from admitting； or in having exaggerated its power； which is in itself probable； I have at least； as I hope； done good service in aiding to overthrow the dogma of separate creations。〃 At the end of the chapter he touches upon the objection as to man's helpless and defenceless condition。 Against this he urges his intelligence and social instincts。
The two following chapters contain a detailed discussion of the objections drawn from the supposed great differences between the mental powers of men and animals。 Darwin at once admits that the differences are enormous； but not that any fundamental difference between the two can be found。 Very characteristic of him is the following passage： 〃In what manner the mental powers were first developed in the lowest organisms； is as hopeless an enquiry as how life itself first originated。 These are problems for the distant future； if they are ever to be solved by man。〃 （Ibid。 page 100。）
After some brief observations on instinct and intelligence； Darwin brings forward evidence to show that the greater number of the emotional states； such as pleasure and pain； happiness and misery； love and hate are common to man and the higher animals。 He goes on to give various examples showing that wonder and curiosity； imitation； attention； memory and imagination （dreams of animals）； can also be observed in the higher mammals； especially in apes。 In regard even to reason there are no sharply defined limits。 A certain faculty of deliberation is characteristic of some animals； and the more thoroughly we know an animal the more intelligence we are inclined to credit it with。 Examples are brought forward of the intelligent and deliberate actions of apes； dogs and elephants。 But although no sharply defined differences exist between man and animals； there is； nevertheless； a series of other mental powers which are characteristics usually regarded as absolutely peculiar to man。 Some of these characteristics are examined in detail； and it is shown that the arguments drawn from them are not conclusive。 Man alone is said to be capable of progressive improvement； but against this must be placed as something analogous in animals； the fact that they learn cunning and caution through long continued persecution。 Even the use of tools is not in itself peculiar to man （monkeys use sticks； stones and twigs）； but man alone fashions and uses implements DESIGNED FOR A SPECIAL PURPOSE。 In this connection the remarks taken from Lubbock in regard to the origin and gradual development of the earliest flint implements will be read with interest； these are similar to the observations on modern eoliths； and their bearing on the development of the stone…industry。 It is interesting to learn from a letter to Hooker （〃Life and Letters〃； Vol。 II。 page 161； June 22； 1859。）； that Darwin himself at first doubted whether the stone implements discovered by Boucher de Perthes were really of the nature of tools。 With the relentless candour as to himself which characterised him； he writes four years later in a letter to Lyell in regard to this view of Boucher de Perthes' discoveries： 〃I know something about his errors； and looked at his book many years ago； and am ashamed to think that I concluded the whole was rubbish！ Yet he has done for man something like what Agassiz did for glaciers。〃 （Ibid。 Vol。 III。 page 15； March 17； 1863。）
To return to Darwin's further comparisons between the higher mental powers of man and animals。 He takes much of the force from the argument that man alone is capable of abstraction and self…consciousness by his own observations on dogs。 One of the main differences between man and animals； speech； receives detailed treatment。 He points out that various animals （birds； monkeys； dogs） have a large number of different sounds for different emotions； that； further； man produces in common with animals a whole series of inarticulate cries combined with gestures； and that dogs learn to understand whole sentences of human speech。 In regard to human language； Darwin expresses a view contrary to that held by Max Muller （〃Descent of Man〃； page 132。）： 〃I cannot doubt that language owes its origin to the imitation and modification of various natural sounds； the voices of other animals； and man's own instinctive cries； aided by signs and gestures。〃 The development of actual language presupposes a higher degree of intelligence than is found in any kind of ape。 Darwin remarks on this point （Ibid。 pages 136； 137。）： 〃The fact of the higher apes not using their vocal organs for speech no doubt depends on their intelligence not having been sufficiently advanced。〃
The sense of beauty； too； has been alleged to be peculiar to man。 In refutation of this assertion Darwin points to the decorative colours of birds； which are used for display。 And to the last objection； that man alone has religion； that he alone has a belief in God； it is answered 〃that numerous races have existed； and still exist； who have no idea of one or more gods； and who have no words in their languages to express such an idea。〃 （Ibid。 page 143。）
The result of the investigations recorded in this chapter is to show that； great as the difference in mental powers between man and the higher animals may be； it is undoubtedly only a difference 〃of degree and not of kind。〃 （〃Descent of Man〃； page 193。）
In the fourth chapter Darwin deals with the MORAL SENSE or CONSCIENCE； which is the most important of all differences between man and animals。 It is a result of social instincts； which lead to sympathy for other members of the same society； to non…egoistic actions for the good of others。 Darwin shows that social tendencies are found among many animals； and that among these love and kin…sympathy exist； and he gives examples of animals （especially dogs） which may exhibit characters that we should call moral in man （e。g。 disinterested self…sacrifice for the sake of others）。 The early ape…like progenitors of the human race were undoubtedly social。 With the increase of intelligence the moral sense develops farther； with the acquisition of speech public opinion arises； and finally； moral sense becomes habit。 The rest of Darw