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erceptible effects； they might； like the colour…factors present in white flowers； make no difference in the form or other characters。 Not till the cross was actually made between the two complementary individuals would either factor come into play； and the effects even then might be unobserved until an attempt was made to breed from the cross…bred。
Next； if the factors responsible for sterility were acquired； they would in all probability be peculiar to certain individuals and would not readily be distributed to the whole breed。 Any member of the breed also into which BOTH the factors were introduced would drop out of the pedigree by virtue of its sterility。 Hence the evidence that the various domesticated breeds say of dogs or fowls can when mated together produce fertile offspring； is beside the mark。 The real question is； Do they ever produce sterile offspring？ I think the evidence is clearly that sometimes they do； oftener perhaps than is commonly supposed。 These suggestions are quite amenable to 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 definit