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experimental tests。 The most obvious way to begin is to get a pair of parents which are known to have had any sterile offspring； and to find the proportions in which these steriles were produced。 If； as I anticipate； these proportions are found to be definite； the rest is simple。
In passing； certain other considerations may be referred to。 First； that there are observations favouring the view that the production of totally sterile cross…breds is seldom a universal property of two species； and that it may be a matter of individuals； which is just what on the view here proposed would be expected。 Moreover； as we all know now； though incompatibility may be dependent to some extent on the degree to which the species are dissimilar； no such principle can be demonstrated to determine sterility or fertility in general。 For example； though all our Finches can breed together； the hybrids are all sterile。 Of Ducks some species can breed together without producing the slightest sterility； others have totally sterile offspring； and so on。 The hybrids between several genera of Orchids are perfectly fertile on the female side； and some on the male side also； but the hybrids produced between the Turnip （Brassica napus） and the Swede （Brassica campestris）； which； according to our estimates of affinity should be nearly allied forms； are totally sterile。 （See Sutton； A。W。； 〃Journ。 Linn。 Soc。〃 XXXVIII。 page 341； 1908。） Lastly； it may be recalled that in sterility we are almost certainly considering a meristic phenomenon。 FAILURE TO DIVIDE is； we may feel fairly sure； the immediate 〃cause〃 of the sterility。 Now； though we know very little about the heredity of meristic differences； all that we do know points to the conclusion that the less…divided is dominant to the more…divided； and we are thus justified in supposing that there are factors which can arrest or prevent cell…division。 My conjecture therefore is that in the case of sterility of cross…breds we see the effect produced by a complementary pair of such factors。 This and many similar problems are now open to our analysis。
The question is sometimes asked； Do the new lights on Variation and Heredity make the process of Evolution easier to understand？ On the whole the answer may be given that they do。 There is some appearance of loss of simplicity； but the gain is real。 As was said above； the time is not ripe for the discussion of the origin of species。 With faith in Evolution unshakenif indeed the word faith can be used in application to that which is certainwe look on the manner and causation of adapted differentiation as still wholly mysterious。 As Samuel Butler so truly said： 〃To me it seems that the 'Origin of Variation；' whatever it is； is the only true 'Origin of Species'〃 （〃Life and Habit〃； London； page 263； 1878。）； and of that Origin not one of us knows anything。 But given Variationand it is given： assuming further that the variations are not guided into paths of adaptationand both to the Darwinian and to the modern school this hypothesis appears to be sound if unprovenan evolution of species proceeding by definite steps is more； rather than less； easy to imagine than an evolution proceeding by the accumulation of indefinite and insensible steps。 Those who have lost themselves in contemplating the miracles of Adaptation （whether real or spurious） have not unnaturally fixed their hopes rather on the indefinite than on the definite changes。 The reasons are obvious。 By suggesting that the steps through which an adaptative mechanism arose were indefinite and insensible； all further trouble is spared。 While it could be said that species arise by an insensible and imperceptible process of variation； there was clearly no use in tiring ourselves by trying to perceive that process。 This labour…saving counsel found great favour。 All that had to be done to develop evolution… theory was to discover the good in everything； a task which； in the complete absence of any control or test whereby to check the truth of the discovery； is not very onerous。 The doctrine 〃que tout est au mieux〃 was therefore preached with fresh vigour； and examples of that illuminating principle were discovered with a facility that Pangloss himself might have envied； till at last even the spectators wearied of such dazzling performances。
But in all seriousness； why should indefinite and unlimited variation have been regarded as a more probable account of the origin of Adaptation？ Only； I think； because the obstacle was shifted one plane back； and so looked rather less prominent。 The abundance of Adaptation； we all grant； is an immense； almost an unsurpassable difficulty in all non…Lamarckian views of Evolution； but if the steps by which that adaptation arose were fortuitous； to imagine them insensible is assuredly no help。 In one most important respect indeed； as has often been observed； it is a multiplication of troubles。 For the smaller the steps； the less could Natural Selection act upon them。 Definite variationsand of the occurrence of definite variations in abundance we have now the most convincing proofhave at least the obvious merit that they can make and often do make a real difference in the chances of life。
There is another aspect of the Adaptation problem to which I can only allude very briefly。 May not our present ideas of the universality and precision of Adaptation be greatly exaggerated？ The fit of organism to its environment is not after all so very closea proposition unwelcome perhaps； but one which could be illustrated by very copious evidence。 Natural Selection is stern； but she has her tolerant moods。
We have now most certain and irrefragable proof that much definiteness exists in living things apart from Selection； and also much that may very well have been preserved and so in a sense constituted by Selection。 Here the matter is likely to rest。 There is a passage in the sixth edition of the 〃Origin〃 which has I think been overlooked。 On page 70 Darwin says 〃The tuft of hair on the breast of the wild turkey…cock cannot be of any use； and it is doubtful 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