rmany as Biologya department of science for which no English word exists except the rather vague term Natural History。 This was especially the case in floral biology； and it is interesting to see with what hesitation he at first expressed the value of his book on Orchids （〃Life and Letters〃； III。 page 254。）； 〃It will perhaps serve to illustrate how Natural History may be worked under the belief of the modification of species〃 （1861）。 And in 1862 he speaks （Loc。 cit。） more definitely of the relation of his work to natural selection： 〃I can show the meaning of some of the apparently meaningless ridges （and） horns； who will now venture to say that this or that structure is useless？〃 It is the fashion now to minimise the value of this class of work； and we even find it said by a modern writer that to inquire into the ends subserved by organs is not a scientific problem。 Those who take this view surely forget that the structure of all living things is； as a whole； adaptive； and that a knowledge of how the present forms come to be what they are includes a knowledge of why they survived。 They forget that the SUMMATION of variations on which divergence depends is under the rule of the environment considered as a selective force。 They forget that the scientific study of the interdependence of organisms is only possible through a knowledge of the machinery of the units。 And that； therefore； the investigation of such widely interesting subjects as extinction and distribution must include a knowledge of function。 It is only those who follow this line of work who get to see the importance of minute points of structure and understand as my father did even in 1842； as shown in his sketch of the 〃Origin〃 （Now being prepared for publication。）； that every grain of sand counts for something in the balance。 Much that is confidently stated about the uselessness of different organs would never have been written if the naturalist spirit were commoner nowadays。 This spirit is strikingly shown in my father's work on the movements of plants。 The circumstance that botanists had not； as a class； realised the interest of the subject accounts for the fact that he was able to gather such a rich harvest of results from such a familiar object as a twining plant。 The subject had been investigated by H。 von Mohl； Palm； and Dutrochet； but they failed not only to master the problem but （which here concerns us） to give the absorbing interest of Darwin's book to what they discovered。
His work on climbing plants was his first sustained piece of work on the physiology of movement； and he remarks in 1864： 〃This has been new sort of work for me。〃 （〃Life and Letters〃； III。 page 315。 He had； however； made a beginning on the movements of Drosera。） He goes on to remark with something of surprise； 〃I have been pleased to find what a capital guide for observations a full conviction of the change of species is。〃
It was this point of view that enabled him to develop a broad conception of the power of climbing as an adaptation by means of which plants are enabled to reach the light。 Instead of being compelled to construct a stem of sufficient strength to stand alone； they succeed in the struggle by making use of other plants as supports。 He showed that the great class of tendril… and root…climbers which do not depend on twining round a pole； like a scarlet…runner； but on attaching themselves as they grow upwards； effect an economy。 Thus a Phaseolus has to manufacture a stem three feet in length to reach a height of two feet above the ground； whereas a pea 〃which had ascended to the same height by the aid of its tendrils； was but little longer than the height reached。〃 （〃Climbing Plants〃 （2nd edition 1875）； page 193。）
Thus he was led on to the belief that TWINING is the more ancient form of climbing； and that tendril…climbers have been developed from twiners。 In accordance with this view we find LEAF…CLIMBERS； which may be looked on as incipient tendril…bearers； occurring in the same genera with simple twiners。 （Loc。 cit。 page 195。） He called attention to the case of Maurandia semperflorens in which the young flower…stalks revolve spontaneously and are sensitive to a touch； but neither of these qualities is of any perceptible value to the species。 This forced him to believe that in other young plants the rudiments of the faculty needed for twining would be founda prophecy which he made good in his 〃Power of Movement〃 many years later。
In 〃Climbing Plants〃 he did little more than point out the remarkable fact that the habit of climbing is widely scattered through the vegetable kingdom。 Thus climbers are to be found in 35 out of the 59 Phanerogamic Alliances of Lindley； so that 〃the conclusion is forced on our minds that the capacity of revolving （If a twining plant； e。g。 a hop； is observed before it has begun to ascend a pole； it will be noticed that； owing to the curvature of the stem； the tip is not vertical but hangs over in a roughly horizontal position。 If such a shoot is watched it will be found that if； for instance； it points to the north at a given hour； it will be found after a short interval pointing north…east； then east； and after about two hours it will once more be looking northward。 The curvature of the stem depends on one side growing quicker than the opposite side； and the revolving movement； i。e。 circumnutation； depends on the region of quickest growth creeping gradually round the stem from south through west to south again。 Other plants； e。g。 Phaseolus； revolve in the opposite direction。）； on which most climbers depend； is inherent； though undeveloped； in almost every plant in the vegetable kingdom。〃 （〃Climbing Plants〃； page 205。）
In the 〃Origin〃 （Edition I。 page 427； Edition VI。 page 374。） Darwin speaks of the 〃apparent paradox； that the very same characters are analogical when one class or order is compared with another； but give true affinities when the members of the same class or order are compared one with another。〃 In this way we might perhaps say that the climbing of an ivy and a hop are analogical； the resemblance depending on the adaptive result rather than on community of blood； whereas the relation between a leaf…climber and a true tendril…bearer reveals descent。 This particular resemblance was one in which my father took especial delight。 He has described an interesting case occurring in the Fumariaceae。 （〃Climbing Plants〃； page 195。） 〃The terminal leaflets of the leaf…climbing Fumaria officinalis are not smaller than the other leaflets； those of the leaf…climbing Adlumia cirrhosa are greatly reduced； those of Corydalis claviculata （a plant which may be indifferently called a leaf…climber or a tendril…bearer） are either reduced to microscopical dimensions or have their blades wholly aborted； so that this plant is actually in a state of transition； and finally in the Dicentra the tendrils are perfectly characterized。〃
It is a remarkable fact that the quality which； broadly speaking； forms the basis of the climbing habit （namely revolving nutation； otherwise known as circumnutation） subserves two distinct ends。 One of these is the finding of a support； and this is common to twiners and tendrils。 Here the value ends as far as tendril…climbers are concerned； but in twiners Darwin believed that the act of climbing round a support is a continuation of the revolving movement （circumnutation）。 If we imagine a man swinging a rope round his head and if we suppose the rope to strike a vertical post； the free end will twine round it。 This may serve as a rough model of twining as explained in the 〃Movements and Habits of Climbing Plants〃。 It is on these pointsthe nature of revolving nutation and the mechanism of twiningthat modern physiologists differ from Darwin。 （See the discussion in Pfeffer's 〃The Physiology of Plants〃 Eng。 Tr。 （Oxford； 1906）； III。 page 34； where the literature is given。 Also Jost； 〃Vorlesungen uber Pflanzenphysiologie〃； page 562； Jena； 1904。）
Their criticism originated in observations made on a revolving shoot which is removed from the action of gravity by keeping the plant slowly rotating about a horizontal axis by means of the instrument known as a klinostat。 Under these conditions circumnutation becomes irregular or ceases altogether。 When the same experiment is made with a plant which has twined spirally up a stick； the process of climbing is checked and the last few turns become loosened or actually untwisted。 From this it has been argued that Darwin was wrong in his description of circumnutation as an automatic change in the region of quickest growth。 When the free end of a revolving shoot points towards the north there is no doubt that the south side has been elongating more than the north； after a time it is plain from the shoot hanging over to the east that the west side of the plant has grown most； and so on。 This rhythmic change of the position of the region of greatest growth Darwin ascribes to an unknown internal regulating power。 Some modern physiologists； however； attempt to explain the revolving movement as due to a particular form of sensitiveness to gravitation which it is not necessary to discuss in detail in this place。 It is sufficient for my purpose to point out that Darwin's explanation of circumnutation is not universally accepted。 Personally I believe that circumnutation is automaticis primarily due to internal stimuli。 It is however in some way connected with gravitational sensitiveness； since the movement normally occurs round a vertical line。 It is not unnatural that； when the plant has no external stimulus by which the vertical can be recognised； the revolving movement should be upset。
Very much the same may be said of the act of twining； namely that most physiologists refuse to accept Darwin's view （above referred to） that twining is the direct result of circumnutation。 Everyone must allow that the two phenomena are in some way connected； since a plant which circumnutates clockwise； i。e。 with the sun； twines in the same direction； and vice versa。 It must also be granted that geotropism has a bearing on the problem； since all plants twine upwards； and cannot twine along a horizontal support。 But how these two factors are combined； and whether any （