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y in the problem of the origin of species； but in all the great problems of biology a new era began。 So unexpected was the discovery that many naturalists were convinced it was untrue； and at once proclaimed Mendel's conclusions as either altogether mistaken； or if true； of very limited application。 Many fantastic notions about the workings of Heredity had been asserted as general principles before： this was probably only another fancy of the same class。
Nevertheless those who had a preliminary acquaintance with the facts of Variation were not wholly unprepared for some such revelation。 The essential deduction from the discovery of segregation was that the characters of living things are dependent on the presence of definite elements or factors； which are treated as units in the processes of Heredity。 These factors can thus be recombined in various ways。 They act sometimes separately； and sometimes they interact in conjunction with each other； producing their various effects。 All this indicates a definiteness and specific order in heredity； and therefore in variation。 This order cannot by the nature of the case be dependent on Natural Selection for its existence； but must be a consequence of the fundamental chemical and physical nature of living things。 The study of Variation had from the first shown that an orderliness of this kind was present。 The bodies and the properties of living things are cosmic； not chaotic。 No matter how low in the scale we go； never do we find the slightest hint of a diminution in that all…pervading orderliness； nor can we conceive an organism existing for a moment in any other state。 Moreover not only does this order prevail in normal forms； but again and again it is to be seen in newly…sprung varieties； which by general consent cannot have been subjected to a prolonged Selection。 The discovery of Mendelian elements admirably coincided with and at once gave a rationale of these facts。 Genetic Variation is then primarily the consequence of additions to； or omissions from； the stock of elements which the species contains。 The further investigation of the species…problem must thus proceed by the analytical method which breeding experiments provide。
In the nine years which have elapsed since Mendel's clue became generally known； progress has been rapid。 We now understand the process by which a polymorphic race maintains its polymorphism。 When a family consists of dissimilar members； given the numerical proportions in which these members are occurring； we can represent their composition symbolically and state what types can be transmitted by the various members。 The difficulty of the 〃swamping effects of intercrossing〃 is practically at an end。 Even the famous puzzle of sex…limited inheritance is solved； at all events in its more regular manifestations； and we know now how it is brought about that the normal sisters of a colour…blind man can transmit the colour…blindness while his normal brothers cannot transmit it。
We are still only on the fringe of the inquiry。 It can be seen extending and ramifying in many directions。 To enumerate these here would be impossible。 A whole new range of possibilities is being brought into view by study of the interrelations between the simple factors。 By following up the evidence as to segregation； indications have been obtained which can only be interpreted as meaning that when many factors are being simultaneously redistributed among the germ…cells； certain of them exert what must be described as a repulsion upon other factors。 We cannot surmise whither this discovery may lead。
In the new light all the old problems wear a fresh aspect。 Upon the question of the nature of Sex； for example； the bearing of Mendelian evidence is close。 Elsewhere I have shown that from several sets of parallel experiments the conclusion is almost forced upon us that； in the types investigated； of the two sexes the female is to be regarded as heterozygous in sex； containing one unpaired dominant element； while the male is similarly homozygous in the absence of that element。 （In other words； the ova are each EITHER female； OR male （i。e。 non…female）； but the sperms are all non…female。） It is not a little remarkable that on this pointwhich is the only one where observations of the nuclear processes of gameto…genesis have yet been brought into relation with the visible characteristics of the organisms themselvesthere should be diametrical opposition between the results of breeding experiments and those derived from cytology。
Those who have followed the researches of the American school will be aware that； after it had been found in certain insects that the spermatozoa were of two kinds according as they contained or did not contain the accessory chromosome； E。B。 Wilson succeeded in proving that the sperms possessing this accessory body were destined to form FEMALES on fertilisation； while sperms without it form males； the eggs being apparently indifferent。 Perhaps the most striking of all this series of observations is that lately made by T。H。 Morgan （Morgan； 〃Proc。 Soc。 Exp。 Biol。 Med。〃 V。 1908； and von Baehr； 〃Zool。 Anz。〃 XXXII。 page 507； 1908。）； since confirmed by von Baehr； that in a Phylloxeran two kinds of spermatids are formed； respectively with and without an accessory （in this case； DOUBLE） chromosome。 Of these； only those possessing the accessory body become functional spermatozoa； the others degenerating。 We have thus an elucidation of the puzzling fact that in these forms fertilisation results in the formation of FEMALES only。 How the males are formedfor of course males are eventually produced by the parthenogenetic femaleswe do not know。
If the accessory body is really to be regarded as bearing the factor for femaleness； then in Mendelian terms female is DD and male is DR。 The eggs are indifferent and the spermatozoa are each male； OR female。 But according to the evidence derived from a study of the sex…limited descent of certain features in other animals the conclusion seems equally clear that in them female must be regarded as DR and male as RR。 The eggs are thus each either male or female and the spermatozoa are indifferent。 How this contradictory evidence is to be reconciled we do not yet know。 The breeding work concerns fowls； canaries； and the Currant moth （Abraxas grossulariata）。 The accessory chromosome has been now observed in most of the great divisions of insects （As Wilson has proved； the unpaired body is not a universal feature even in those orders in which it has been observed。 Nearly allied types may differ。 In some it is altogether unpaired。 In others it is paired with a body of much smaller size； and by selection of various types all gradations can be demonstrated ranging to the condition in which the members of the pair are indistinguishable from each other。）； except； as it happens； Lepidoptera。 At first sight it seems difficult to suppose that a feature apparently so fundamental as sex should be differently constituted in different animals； but that seems at present the least improbable inference。 I mention these two groups of facts as illustrating the nature and methods of modern genetic work。 We must proceed by minute and specific analytical investigation。 Wherever we look we find traces of the operation of precise and specific rules。
In the light of present knowledge it is evident that before we can attack the Species…problem with any hope of success there are vast arrears to be made up。 He would be a bold man who would now assert that there was no sense in which the term Species might not have a strict and concrete meaning in contradistinction to the term Variety。 We have been taught to regard the difference between species and variety as one of degree。 I think it unlikely that this conclusion will bear the test of further research。 To Darwin the question； What is a variation？ presented no difficulties。 Any difference between parent and offspring was a variation。 Now we have to be more precise。 First we must； as de Vries has shown； distinguish real； genetic； variation from FLUCTUATIONAL variations； due to environmental and other accidents； which cannot be transmitted。 Having excluded these sources of error the variations observed must be expressed in terms of the factors to which they are due before their significance can be understood。 For example； numbers of the variations seen under domestication； and not a few witnessed in nature； are simply the consequence of some ingredient being in an unknown way omitted from the composition of the varying individual。 The variation may on the contrary be due to the addition of some new element； but to prove that it is so is by no means an easy matter。 Casual observation is useless； for though these latter variations will always be dominants； yet many dominant characteristics may arise from another cause； namely the meeting of complementary factors； and special study of each case in two generations at least is needed before these two phenomena can be distinguished。
When such considerations are fully appreciated it will be realised that medleys of most dissimilar occurrences are all confused together under the term Variation。 One of the first objects of gene