September 3, 1914] 



NATURE 



23 



together in the zygote? The result would surely 

 depend upon the interaction of the complex colloidal 

 multimolecules of which the "chromatin is composed. 

 \ arious possibilities would arise, (i) The two samples 

 might dilTer in such a way as to act as poisons to 

 one another, disturbing eacn other's molecular equili- 

 brium to such an extent that neither could survive. 

 This is possibly what happens when an ovum is fer- 

 tilised by a spermatozoon of a distinct species, though 

 there are, ot course, exceptions. (2) They might be 

 ?o alike as to be able to amalgamate more or less 

 completely, so that there would simply be an increase 

 of chromatin of possibly more or less modified con- 

 stitution. (3) Ihey might continue to exist side by 

 side, each maintaining its own individual character. 



In the third case the union of the two different 

 -umples would give rise to a mass of chromatin of 

 .vofold nature, and repetition of the process from 

 :^eneration to generation would, as \\eismann has 

 -hown result in ever-increasing heterogeneity, until 

 the chromatin came to consist of a great number of 

 dilierent concrete particles, each of which might con- 

 ceivably differ from all the others. But when two 

 heterogeneous masses of chromatin meet in the zygote 

 there may be all sorts of mutual attractions and 

 repulsions between the different colloidal multimole- 

 cules, for all three of our supposed cases may arise 

 simultaneously, and thus the results may become 

 extremely complicated. 



The chromatin of the germ-cells in all existing 

 organisms is undoubtedly heterogeneous, and this 

 heterogeneity may be to some extent visiblj- expressed 

 in its arrangement in more or less multiform chromo- 

 somes during mitosis. We may provisionally accept 

 W'eismann's view that these chromosomes are them- 

 selves heterogeneous, being composed of chromomeres 

 r ids, whicn in their turn are composed of deter- 

 minants. 



All this complexity of structure may be attributed 

 to the effects of oft-repeated amphimixis, a view which 

 is supported in the most striking manner by the fact 

 that the nucleus in all ordinar}- somatic cells (in 

 nimals and in the diploid generation of plants) has 

 :. double set of chromosomes, one derived from the 

 male and the other from the female parent, and by 

 the well-known phenomenon of chromatin reduction 

 which always precedes amphimixis. 



When we approach the problem of heredity from 

 the experimental side we get very strong evidence of 

 the existence in the germ-plasm of definite material 

 -ubstances associated with the inheritance of special 

 haracters. Mendelian workers generally speak of 

 liiese substances as factors, but the conception of 

 factors is evidently closely akin to that of Weismann's 

 hypothetical determinants. The cytological evidence 

 fits in ven,' well with the view that the factors in 

 question may be definite material particles, and it is 

 quite possible that such particles may have a specific 

 chemical constitution to which their effects upon the 

 developing organism are due. 



From our point of view the interesting thing is the 

 possibility that arises through the sexual process of 

 the permutation and combination of different factors 

 derived from different lines of descent. A germ-cell 

 may receive additions to its collection of factors or 

 be subject to subtractions therefrom, and in either case 

 the resulting organism may be more or less con- 

 spicuously modified. 



By applyiner the method of experimental hvbridisa- 

 tion a most fruitful and apparentlv inexhaustible field 

 of research has been opened up in this direction, in 

 f^he^ development of which no one has taken a more 



tive part than the present President of the British 

 Association. There cannot be the slightest doubt that 



NO. 2340, VOL. 94I 



a vast number of characters are inherited in what is 

 called the Mendelian manner, and, as they are capable 

 of being separately inherited and interchanged with 

 others by hybridisation, we are justified in believing 

 that they are separately represented in the germ-cells 

 by special factors. Important as this result is, I 

 believe that at the present time there exists a distinct 

 danger of exaggerating its significance. The fact that 

 many new and apparently permanent combinations of 

 charaicters may arise through hybridisation, and that 

 the organisms thus produced have all the attributes of 

 what we call distinct species, does not justifv us in 

 accepting the grotesque view — as it appears to me — 

 that all species have arisen by crossing, or even the 

 view that the organism is entirely built up of separately 

 transmissible "unit characters." 



Bateson tells us that " Baur has for example 

 crossed species so unlike as Antirrhinum majus and 

 molle, forms differing from each other in almost ever>' 

 feature of organisation.*' Surelv the latter part of 

 this statement cannot be correct, for after all Antir- 

 rhinum majus and molle are both snapdragons, and 

 exhibit all the essential characters of snapdragons. 



I think it is a most significant fact that the only 

 characters which appear to be inherited in Mendelian 

 fashion are comparatively trivial features of the 

 organism which must have arisen during the last 

 stages of phylogeny. This is necessarily the case, for 

 anv two organisms sufficiently nearly related to be 

 capable of crossing are identical as regards the vast 

 majority- of their characters. It is only those few 

 points in which they differ that remain to be experi- ' 

 men ted on. Moreover, the characters in question 

 appear to be all non-adaptive, having no obvious rela- 

 tion to the environment and no particular value in 

 the struggle for existence. They are clearly what 

 Weismann calls blastogenic characters, originating 

 in the germ-plasm, and are probably identical with 

 the mutations of de Vries. These latter are ap- 

 parently chromatin-determined characters, for, as Dr. 

 Gates has recently shown in the case of CEnothera, 

 mutation may result from abnormal distribution of the 

 chromosomes in the reduction division.* 



We have next to inquire whether or not the Men- 

 delian results are really in any way inconsistent with 

 the general theorv of evolution outlined in the earlier 

 part of this address. Here we are obviously face to 

 face with the old dispute between epigenesis and pre- 

 formation. The theon,- of ontogeny which I first put 

 forward is clearly epigenetic in character, while the 

 theor>' of unit characters, represented in the germ- 

 cells bv separate • factors," is scarcely less clearlv a 

 theon.- of preformation, and of course the conception 

 of definite organ-forming substances in the cytoplasm 

 falls under the same category. The point which I 

 now wish to emphasise is that the ideas of epigenesis 

 and preformation are not inconsistent with one 

 another, and that, as a matter of fact, ontogenetic 

 development is of a dual nature, an epigenesis 

 modified bv what is essentially preformation. 



We have already dealt briefly with the question of 

 organ-forming substances in the cytoplasm, and it 

 must, I think, be clear that the existence of these is 

 in no wav incompatible with a fundamental epi- 

 genesis. We shall find directly that the same is true 

 of Mendelian "factors" or Weismannian "deter- 

 minants." 



We have seen that it is possible to conceive of even 

 a complex organism as inheriting nothing from its 

 parent but a minute speck of protoplasm, endowed 

 with potential energ>-, and a sequence of suitable 

 environments, the interaction between the two bring- 

 ing about a similar result in each succeeding genera- 



« Qnarterly J cuf-nal t>/ Microscopical Science, vol. lix., p. SS7 



