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l whether it can be ornamental in the eyes of the female bird。〃 This tuft of hair is a most definite and unusual structure； and I am afraid that the remark that it 〃cannot be of any use〃 may have been made inadvertently； but it may have been intended； for in the first edition the usual qualification was given and must therefore have been deliberately excised。 Anyhow I should like to think that Darwin did throw over that tuft of hair； and that he felt relief when he had done so。 Whether however we have his great authority for such a course or not； I feel quite sure that we shall be rightly interpreting the facts of nature if we cease to expect to find purposefulness wherever we meet with definite structures or patterns。 Such things are； as often as not； I suspect rather of the nature of tool…marks； mere incidents of manufacture； benefiting their possessor not more than the wire…marks in a sheet of paper； or the ribbing on the bottom of an oriental plate renders those objects more attractive in our eyes。
If Variation may be in any way definite； the question once more arises； may it not be definite in direction？ The belief that it is has had many supporters； from Lamarck onwards； who held that it was guided by need； and others who； like Nageli； while laying no emphasis on need； yet were convinced that there was guidance of some kind。 The latter view under the name of 〃Orthogenesis；〃 devised I believe by Eimer； at the present day commends itself to some naturalists。 The objection to such a suggestion is of course that no fragment of real evidence can be produced in its support。 On the other hand； with the experimental proof that variation consists largely in the unpacking and repacking of an original complexity； it is not so certain as we might like to think that the order of these events is not pre…determined。 For instance the original 〃pack〃 may have been made in such a way that at the nth division of the germ…cells of a Sweet Pea a colour…factor might be dropped； and that at the n plus n prime division the hooded variety be given off； and so on。 I see no ground whatever for holding such a view； but in fairness the possibility should not be forgotten； and in the light of modern research it scarcely looks so absurdly improbable as before。
No one can survey the work of recent years without perceiving that evolutionary orthodoxy developed too fast； and that a great deal has got to come down； but this satisfaction at least remains； that in the experimental methods which Mendel inaugurated； we have means of reaching certainty in regard to the physiology of Heredity and Variation upon which a more lasting structure may be built。
VI。 THE MINUTE STRUCTURE OF CELLS IN RELATION TO HEREDITY。
By EDUARD STRASBURGER； Professor of Botany in the University of Bonn。
Since 1875 an unexpected insight has been gained into the internal structure of cells。 Those who are familiar with the results of investigations in this branch of Science are convinced that any modern theory of heredity must rest on a basis of cytology and cannot be at variance with cytological facts。 Many histological discoveries； both such as have been proved correct and others which may be accepted as probably well founded； have acquired a fundamental importance from the point of view of the problems of heredity。
My aim is to describe the present position of our knowledge of Cytology。 The account must be confined to essentials and cannot deal with far… reaching and controversial questions。 In cases where difference of opinion exists； I adopt my own view for which I hold myself responsible。 I hope to succeed in making myself intelligible even without the aid of illustrations： in order to convey to the uninitiated an adequate idea of the phenomena connected with the life of a cell； a greater number of figures would be required than could be included within the scope of this article。
So long as the most eminent investigators （As for example the illustrious Wilhelm Hofmeister in his 〃Lehre von der Pflanzenzelle〃 （1867）。） believed that the nucleus of a cell was destroyed in the course of each division and that the nuclei of the daughter…cells were produced de novo； theories of heredity were able to dispense with the nucleus。 If they sought； as did Charles Darwin； who showed a correct grasp of the problem in the enunciation of his Pangenesis hypothesis； for histological connecting links； their hypotheses； or at least the best of them； had reference to the cell as a whole。 It was known to Darwin that the cell multiplied by division and was derived from a similar pre…existing cell。 Towards 1870 it was first demonstrated that cell…nuclei do not arise de novo； but are invariably the result of division of pre…existing nuclei。 Better methods of investigation rendered possible a deeper insight into the phenomena accompanying cell and nuclear divisions and at the same time disclosed the existence of remarkable structures。 The work of O。 Butschli； O。 Hertwig； W。 Flemming H。 Fol and of the author of this article （For further reference to literature； see my article on 〃Die Ontogenie der Zelle seit 1875〃； in the 〃Progressus Rei Botanicae〃； Vol。 I。 page 1； Jena； 1907。）； have furnished conclusive evidence in favour of these facts。 It was found that when the reticular framework of a nucleus prepares to divide； it separates into single segments。 These then become thicker and denser； taking up with avidity certain stains； which are used as aids to investigation； and finally form longer or shorter； variously bent； rodlets of uniform thickness。 In these organs which； on account of their special property of absorbing certain stains； were styled Chromosomes （By W。 Waldeyer in 1888。）； there may usually be recognised a separation into thicker and thinner discs； the former are often termed Chromomeres。 （Discovered by W。 Pfitzner in 1880。） In the course of division of the nucleus； the single rows of chromomeres in the chromosomes are doubled and this produces a band…like flattening and leads to the longitudinal splitting by which each chromosome is divided into two exactly equal halves。 The nuclear membrane then disappears and fibrillar cell…plasma or cytoplasm invades the nuclear area。 In animal cells these fibrillae in the cytoplasm centre on definite bodies （Their existence and their multiplication by fission were demonstrated by E。 van Beneden and Th。 Boveri in 1887。）； which it is customary to speak of as Centrosomes。 Radiating lines in the adjacent cell…plasma suggest that these bodies constitute centres of force。 The cells of the higher plants do not possess such individualised centres； they have probably disappeared in the course of phylogenetic development： in spite of this； however； in the nuclear division…figures the fibrillae of the cell…plasma are seen to radiate from two opposite poles。 In both animal and plant cells a fibrillar bipolar spindle is formed； the fibrillae of which grasp the longitudinally divided chromosomes from two opposite sides and arrange them on the equatorial plane of the spindle as the so… called nuclear or equatorial plate。 Each half…chromosome is connected with one of the spindle poles only and is then drawn towards that p