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r of to…day； or of yesterday； or of ages long gone by； every kind of cell； whether glandular； muscular； nervous； epidermic； or skeletal； is adapted to absolutely definite and specific functions； and every organ which is composed of these different kinds of cells contains them in the proper proportions； and in the particular arrangement which best serves the function of the organ； it is thus adapted to its function。
All parts of the organism are tuned to one another； that is； THEY ARE ADAPTED TO ONE ANOTHER； and in the same way THE ORGANISM AS A WHOLE IS ADAPTED TO THE CONDITIONS OF ITS LIFE； AND IT IS SO AT EVERY STAGE OF ITS EVOLUTION。
But all adaptations CAN be referred to selection； the only point that remains doubtful is whether they all MUST be referred to it。
However that may be； whether the LAMARCKIAN PRINCIPLE is a factor that has cooperated with selection in evolution； or whether it is altogether fallacious； the fact remains； that selection is the cause of a great part of the phyletic evolution of organisms on our earth。 Those who agree with me in rejecting the LAMARCKIAN PRINCIPLE will regard selection as the only GUIDING factor in evolution； which creates what is new out of the transmissible variations； by ordering and arranging these； selecting them in relation to their number and size； as the architect does his building… stones so that a particular style must result。 （〃Variation under Domestication〃； 1875 II。 pages 426； 427。） But the building…stones themselves； the variations； have their basis in the influences which cause variation in those vital units which are handed on from one generation to another； whether； taken together they form the WHOLE organism； as in Bacteria and other low forms of life； or only a germ…substance； as in unicellular and multicellular organisms。 （The Author and Editor are indebted to Professor Poulton for kindly assisting in the revision of the proof of this Essay。）
IV。 VARIATION。
By HUGO DE VRIES； Professor of Botany in the University of Amsterdam。
I。 DIFFERENT KINDS OF VARIABILITY。
Before Darwin； little was known concerning the phenomena of variability。 The fact； that hardly two leaves on a tree were exactly the same； could not escape observation： small deviations of the same kind were met with everywhere； among individuals as well as among the organs of the same plant。 Larger aberrations； spoken of as monstrosities； were for a long time regarded as lying outside the range of ordinary phenomena。 A special branch of inquiry； that of Teratology； was devoted to them； but it constituted a science by itself； sometimes connected with morphology； but having scarcely any bearing on the processes of evolution and heredity。
Darwin was the first to take a broad survey of the whole range of variations in the animal and vegetable kingdoms。 His theory of Natural Selection is based on the fact of variability。 In order that this foundation should be as strong as possible he collected all the facts； scattered in the literature of his time； and tried to arrange them in a scientific way。 He succeeded in showing that variations may be grouped along a line of almost continuous gradations； beginning with simple differences in size and ending with monstrosities。 He was struck by the fact that； as a rule； the smaller the deviations； the more frequently they appear； very abrupt breaks in characters being of rare occurrence。
Among these numerous degrees of variability Darwin was always on the look out for those which might； with the greatest probability； be considered as affording material for natural selection to act upon in the development of new species。 Neither of the extremes complied with his conceptions。 He often pointed out； that there are a good many small fluctuations； which in this respect must be absolutely useless。 On the other hand； he strongly combated the belief； that great changes would be necessary to explain the origin of species。 Some authors had propounded the idea that highly adapted organs； e。g。 the wings of a bird； could not have been developed in any other way than by a comparatively sudden modification of a well defined and important kind。 Such a conception would allow of great breaks or discontinuity in the evolution of highly differentiated animals and plants； shortening the time for the evolution of the whole organic kingdom and getting over numerous difficulties inherent in the theory of slow and gradual progress。 It would； moreover； account for the genetic relation of the larger groups of both animals and plants。 It would； in a word； undoubtedly afford an easy means of simplifying the problem of descent with modification。
Darwin； however； considered such hypotheses as hardly belonging to the domain of science； they belong； he said； to the realm of miracles。 That species have a capacity for change is admitted by all evolutionists； but there is no need to invoke modifications other than those represented by ordinary variability。 It is well known that in artificial selection this tendency to vary has given rise to numerous distinct races； and there is no reason for denying that it can do the same in nature； by the aid of natural selection。 On both lines an advance may be expected with equal probability。
His main argument； however； is that the most striking and most highly adapted modifications may be acquired by successive variations。 Each of these may be slight； and they may affect different organs； gradually adapting them to the same purpose。 The direction of the adaptations will be determined by the needs in the struggle for life； and natural selection will simply exclude all such changes as occur on opposite or deviating lines。 In this way； it is not variability itself which is called upon to explain beautiful adaptations； but it is quite sufficient to suppose that natural selection has operated during long periods in the same way。 Eventually； all the acquired characters； being transmitted together； would appear to us； as if they had all been simultaneously developed。
Correlations must play a large part in such special evolutions： when one part is modified； so will be other parts。 The distribution of nourishment will come in as one of the causes； the reactions of different organs to the same external influences as another。 But no doubt the more effective cause is that of the internal correlations； which； however； are still but dimly understood。 Darwin repeatedly laid great stress on this view； although a definite proof of its correctness could not be given in his time。 Such proof requires the direct observation of a mutation； and it should be stated here that even the first observations made in this direction have clearly confirmed Darwin's ideas。 The new evening primroses which have sprung in my garden from the old form of Oenothera Lamarckiana； and which have evidently been derived from it； in each case； by a single mutation； do not differ from their parent species in one character only； but in almost all their organs and qualities。 Oenothera gigas； for example； has stouter stems and denser foliage； the leaves are larger and broader； its thick flower…buds produce gigantic flowers； but only small fruits with l