August 25, 1904] 



NA TURE 



409 



Sclcclion will never make the blues breed true : nor can 

 this ever come to pass unless a blue be found the germ-cells 

 of which are bearers of the blue character — which may or 

 niav not be possible. If the selectionist reflect on this 

 experience he will be led straight to the centre of our 

 problem. There will fall, as it were, scales from his eyes, 

 and in a flash he will see the true meaning of fixation of 

 type, variability, and mutation, vaporous mysteries no more. 



Owing to the unhappy subdivisions of our studies, such 

 phenomena as these — constant companions of the breeder — 

 come seldom within the purview of modern science, which, 

 forced for a moment to contemplate them, e.xpresses astonish- 

 ment and relapses into indolent scepticism. It is in the 

 hope that a little may be done to draw research back into 

 these forgotten paths that I avail myself of this great oppor- 

 tunity of speaking to my colleagues with somewhat wider 

 range of topic than is possible within the limits of a scientific 

 paper. For I am convinced that the investigation of heredity 

 by experimental methods offers the sole chance of progress 

 with the fundamental problems of evolution. 



In saying this I mean no disrespect to that study of the 

 physiology of reproduction by histological means, which, 

 largely through the stimulus of Weismann's speculations, 

 has of late made such extraordinary advances. It needs no 

 penetration to see that, by an exact know-ledge of the pro- 

 cesses of maturation and fertilisation, a vigorous stock is 

 being reared, upon which some day the experience of the 

 breeder will be firmly grafted, to our mutual profit. We, 

 who are engaged in experimental breeding, are watching 

 with keenest interest the researches of .Strasburger, Boveri, 

 Wilson, Farmer, and their many fellow-workers and 

 associates in this diflicult field, sure that in the near future 

 we shall be operating in common. We know already that 

 the experience of the breeder is in no way opposed to the 

 facts of the histologist ; but the point at which we shall 

 unite will be found when it is possible to trace in the 

 maturing germ an indication of some character afterwards 

 recognisable in the resulting organism. Until then, in order 

 to pursue directly the course of heredity and variation, it 

 is evident that we must fall back on those tangible manifest- 

 ations which are to be studied only by field observation and 

 experimental breeding. 



The breeding-pen is to us what the test-tube is to the 

 chemist — an instrument whereby we examine the nature of 

 our organisms and determine empirically what for brevity 

 I may call their genetic properties. As unorganised sub- 

 stances have their definite properties, so have the several 

 species and varieties which form the materials of our ex- 

 periments. Every attempt to determine these definite 

 properties contributes immediately to the solution of that 

 problem of problems, the physical constitution of a living 

 organism. In those morphological studies which I suppose 

 most of us have in our time pursued, we sought inspiration 

 from the belief that in the examination of present normalities 

 we were tracing the past, the phylogenetic order of our 

 types, the history — as we conceived — of Evolution. In the 

 w-ork which I am now pressing upon your notice we may 

 claim to be dealing not only with the present and the past, 

 but with the future also. 



On such an occasion as this it is impossible to present 

 to you in detail the e.xperiments — some exceedingly complex 

 — already made in response to this newer inspiration. I must 

 speak of results, not of methods. At a later meeting, more- 

 over, there will be opportunities of exhibiting practically 

 to those interested some of the more palpable illustrations. 

 It is also impossible to-day to make use of the symbolic 

 demonstrations by which the lines of analysis must be re- 

 presented. The time cannot be far distant when ordinary 

 Mendelian formul.Tc will be mere as in praesenti to a 

 biological audience. Xearly five years have passed since 

 this extraordinary re-discovery was made known to the 

 scientific world by the practically simultaneous papers of 

 De Vries, Correns, and Tschermak, not to speak of thirty- 

 fivp years of neglect endured before. Vet a phenomenon 

 comparable in significance w'ith any that biological science 

 has revealed remains the intellectual possession of specialists. 

 We still speak sometimes of Mendel's hypothesis or theory, 

 but in truth the terms have no strict application. It is no 

 theory that water is made up of hydrogen and oxygen, 

 though we cannot watch the atoms unite, and it is no theory 

 that the blue Andalusian fowl I produce was made by the 



NO. 18 I 7, VOL. 70] 



meeting of germ-cells bearing respectively black and a 

 peculiar white. Both are incontrovertible facts deduced 

 from observation. The two facts have this in common also, 

 that their perception gives us a glimpse into that hidden 

 order out of which the seeming disorder of our world is 

 built. If I refer to Mendelian " theory," therefore, in the 

 words with which Bacon introduced his Great Instauration, 

 " I entreat men to believe that it is not an opinion to be 

 held, but a work to be done ; and to be well assured that I 

 am labouring to lay the foundation, not of any sect or 

 doctrine, but of human utility and power." 



In the Mendelian method of experiment the one essential 

 is that the posterity of each individual should be traced 

 separately. If individuals from necessity are treated 

 collectively, it must be proved that their composition is 

 identical. In direct contradiction to the methods of current 

 statistics, Mendel saw by sure penetration that masses must 

 be avoided. Obvious as this necessity seems when one is 

 told, no previous observer had thought of it, whereby the 

 discoverv was missed. As Mendel itnmediately proved in 

 the case of peas, and as we have now seen in many other 

 plants and animals, it is often impossible to distinguish by 

 inspection individuals whose genetic properties are totally 

 distinct. Breeding gives the only test. 



Segregation. 



Where the proper precautions have been taken, the follow- 

 ing phenomena have been proved to occur in a great range 

 of cases, affecting many characters in some thirty plants 

 and animals. The qualities or characters the transmission 

 of which in heredity is examined are found to be distributed 

 among the germ-cells, or gametes, as they are called, 

 according to a definite system. This system is such that 

 these characters are treated by the cell-divisions (from which 

 the gametes result) as existing in pairs, each member of 

 a pair being alternative or alleloinorphic to the other in the 

 composition of the germ. Now, as every zygote — that is, 

 any ordinary animal or plant — is formed by the union of 

 two gametes, it may either be made by the union of two 

 gametes bearing similar members of any pair, say two 

 blacks or two whites, in which case we call it homozygous 

 in respect of that pair, or the gametes from which it 

 originates may be bearers of the dissimilar characters, say 

 a black and a white, when we call the resulting zygote 

 heterozygous in respect of that pair. If the zygote is homo- 

 zygous, no matter what its parents or their pedigree may 

 have been, it breeds true indefinitely unless some fresh 

 variation occurs. 



If, however, the zygote be heterozygous, or gametically 

 cross-bred, its gametes in their formation separate the 

 allelomorphs again, so that each gamete contains only one 

 allelomorphic character of each pair. At least one cell- 

 division in the process of gametogenesis is therefore a 

 differentiating or segregating division, out of which each 

 gamete comes sensibly pure in respect of the allelomorph 

 it carries, exactly as if it had not been formed by a hetero- 

 zygous body at all. That, translated into modern language, 

 is the essential discovery that Mendel made. It has now 

 been repeated and verified for numerous characters of 

 numerous spepies, and, in face of heroic efforts to shake the 

 evidence or to explain it away, the discovery of gametic 

 segregation is, and will remain, one of the lasting triumphs 

 of the human mind. 



In extending our acquaintance of these phenomena of 

 segregation we encounter several principal types of com- 

 plication. 



Segregation Absent or Incomplete. — From our general 

 knowledge of breeding we feel fairly well satisfied that true 

 absence of segregation is the rule in certain cases. It is 

 difficult, for instance, to imagine any other account of the 

 facts respecting the American Mulattos, though even here 

 sporadic occurrence of segregation seems to be authenticated. 

 Very few instances of genuine absence of segregation have 

 been critically studied. The only one I can cite from my 

 own experience is that of Pararge egeria and egeriades, 

 " climatic " races of a butterfly. When crossed together, 

 they give the common intermediate type of North-Western 

 France, which, though artificially formed, breeds in great 

 measure true. This crossed back with either type has 

 given, as a rule, simple blends between intermediate and 

 type. My evidence is not, however, complete enough to 



