322 



NATURE 



[Feb. 6, 1890 



An interesting and important modification of Boveri's 

 experiment confirmed both this experiment, and also, if it 

 were necessary, the recognition of the nuclear substance 

 as idioplasm, as maintained by O. Hertwig, Strasburger, 

 and myself. If eggs of Echinus micro tuberculatus, when 

 artificially deprived of their nuclei, be fertilized with the 

 spermatozoa of SphcBrechimis granulatus, larvce are de- 

 veloped with the true characters of the second species — 

 that is to say, they have derived everything from the 

 father, nothing from the mother ; the nuclear substance 

 alone it is which transmits heredity, and by it the cell-mass 

 is dominated. 



I have interpreted the first polar body of the Metazoan 

 ovum as a carrier of ovogenous plasm, which has to be 

 removed from the ovum in order that the germ-plasm 

 may attain the predominance. It is possible that this 

 explanation is not correct ; the most recent researches 

 on the conjugation of Infusoria, as expressed in the 

 splendid memoirs of Maupas and R. Hertwig, argue 

 against my interpretation ; but the idea which lay at the 

 bottom of this explanation is justified. As it is the nu- 

 clear matter which gives to the cell-body its specific 

 character, the ovum must, previous to fertilization, be 

 dominated by a different idioplasm to the sperm-cell, 

 since they are, up to this point, different in appearance 

 and function. On the other hand, when they have 

 united, they contain the same idioplasm — namely, germ- 

 plasm ; the consequence is that the first dominant idio- 

 plasm is different to that of a later period. This was the 

 idea at the bottom of my explanation of the first polar 

 body, and it is correct. One might perhaps imagine that 

 the idioplasmata of ovum and spermatozoon were origin- 

 ally different, but that both possessed the power of 

 alteration into germ-plasm ; but it would be then incom- 

 prehensible why parthenogenetic ova should expel one 

 polar body. Both facts, however, are explicable, if ovum 

 and spermatozoon are dominated up to the period of 

 maturation by different histogenetic idioplasmata with 

 which a small quantity of germ-plasm is mingled, and if 

 at a later period the former be removed and the germ- 

 plasm come to rule in both cells. This process would be 

 by no means abnormal and unparalleled, since entirely 

 analogous divisions of the idioplasm into qualitatively 

 dissimilar portions must occur hundreds of times in every 

 embryogenesis. However, I am most willing to allow 

 that the last word has not yet been said on this question, 

 and would only maintain that my theory of heredity is 

 not concerned thereby. It is not the interpretation of 

 the first polar body, but that of the second, which is de- 

 cisive ; and one can none the less easily think of the latter 

 as a halving of the number of ancestral germ-plasmata, 

 even if it be proved that my explanation of the first polar 

 body was erroneous. I would then express the first 

 division merely as introductory to the second, as the 

 necessary first step in the reduction of ancestral plasmata, 

 the necessity for which we should thus perhaps learn to 

 understand. 



The regular modification of idioplasma during the 

 ontogeny, which I have maintained and which so many 

 have attacked (Kollikeri with special vehemence) will 

 now stand out as justified. If the nucleus of a sperm-cell 

 is capable of impressing on the denucleated mass of an 

 ovum its own inherited tendencies, and of calling into 

 being an organism with specific characteristics purely 

 paternal, it will be found difficult to explain the ontogeny 

 otherwise than as a regular modification of the idioplasm, 

 continuous from one cell-division to another, which stamps 

 on the body of each separate cell at each stage its peculiar 

 character, not only with regard to shape but also to 

 function, and especially with regard to the " rhythm " of 

 cell-division. 



' " Das Karyoplasma und die Vererbung : eine Kritik der Weismann'sche 

 Theorie von der Continui'at des Keimplasma's," Zcit. whs. ZocL, xliv. 

 p. 228, 1886. 



A further objection is directed by Prof. Vines against 

 my views on the origin of variation. In the fifth essay I 

 have sought the significance of sexual reproduction in the 

 fact that it alone could have called into existence that 

 multiplicity of form of the higher animals and plants, and 

 that constantly fluctuating union of individual variations, 

 of which natural selection stood in need for the creation 

 of new species. I am still of the opinion that the origin 

 of sexual reproduction depends on the advantage which 

 it affords to the operation of natural selection ; nay, I am 

 completely convinced that only through its introduction 

 was the higher development of the organic world possible. 

 Still, I am at present inclined to believe that Prof. Vines 

 is correct in questioning whether sexual reproduction is 

 the only factor which maintains Metazoa and Metaphyta 

 in a state of variability. I could have pointed out in the 

 English edition of my "Essays" that my views on this point 

 had altered since their publication ; my friend Prof, de 

 Bary, too early lost to science, had already called my 

 attention to those parthenogenetic Fungi which Prof. 

 Vines justly cites against my views ; but I desired, on 

 grounds already mentioned, to undertake no alteration in 

 the essays. Bessides, I was well aware when the essay 

 was first committed to paper (1886) that my current view 

 on the radical cause of variation was possibly incomplete ; 

 and so, in order to expose the truth of the view as far as 

 possible to a general test, I drove its logical consequences 

 home, and enunciated the statement that species repro- 

 ducing parthenogenetically could not be modified into 

 new species. I also began myself at that time experi- 

 ments on the variation of parthenogenetic species which 

 are still being continued, and on which on some future 

 occasion I hope to be able to report. 



Even if, however, from our present knowledge it is 

 probable that sexual reproduction is not the sole radical 

 cause of variability of the Metazoa, still no one will dispute 

 that it is a most active means of heightening variations 

 and of mingling them in favourable proportions. I believe 

 that the important part which this method of reproduction 

 has played in calling out the existing processes of selection, 

 is hardly diminished, even if one grants that direct influ- 

 ences upon the idioplasm call forth a portion of individual 

 variability. Prof. Vines even holds it probable "that 

 the absence of sexuality in these plants [Fungi] may 

 be just the reason why no higher forms have been evolved 

 from them, for in this respect they present a striking 

 contrast to the higher Algse in which sexuality is well 

 marked." But when Prof. Vines says, " there can be 

 no doubt that sexual reproduction does very materially 

 promote variation," he does not mean to say that this is 

 a self-evident proposition ; he is well aware that promi- 

 nent investigators like Strasburger see in sexual reproduc- 

 tion the reverse action, that of maintaining the constancy 

 of the specific character. But I gladly accept his agree- 

 ment with my view, which confirms the main position of 

 the fifth essay, which runs : Sexual reproduction has 

 arisen by and for natural selection as the sole means by 

 which individual variations can be united and combined 

 in every possible proportion. 



With reference also to the problem of the inheritance 

 of acquired (somatogenic) characters. Prof. Vines is 

 again my opponent ; he holds that such inheritance is 

 possible. I have denied it, because it did not appear to 

 me self-evident— as was formerly universally assumed — 

 but rather utterly unproven ; and because I think that 

 completely unfounded assumptions of such far-reaching 

 consequence should not be made, when requiring a large 

 number of improbable hypotheses for their exphcation. I 

 have tested all the available evidence for such inheritance 

 as accurately as I could, and have found that none has 

 the value of proof. There is no inheritance of mutilations, 

 and this constitutes up to now the only basis of fact for 

 the supposition of the inheritance of somatogenic varia- 

 tions. If, in the last essay, I have not denied every 



