NA TURK 



205 



THURSDAY, JANUARY 17, 1895. 



EPIGENESIS OR EVOLUTION. 

 2eit- iind Straitfragen der Biologic. Von Prof. Dr. 

 Oscar Hertwig. Heft i : Praforraation oder Epi- 

 genese ? GrundzUge einer Entwicklungs-theorie der 

 ' Organismen. (Jena: Gustav Fischer, 1894.) 



' I 'HE theory of preformation, or rather let us say pre- 

 i J- determination, as revived at the close of the nine- 

 i teenth century, is much more formidable than its prototype 

 j of the eighteenth century. Not only is it stripped of all its 

 j «arlier crudities, such as the doctrine of c-mboilement, but 

 i it is supported by a mass of evidence accumulated by the 

 researches of the last quarter of a century. Its funda- 

 mental assumption, that of the existence of minute, 

 qualitatively unlike, ultimate particles of living matter, 

 is strengthened, if not supported, by the analogy of 

 the atomic theory, and the observed phenomena of 

 mitosis accompanying the maturation of the ovum 

 and spermatozoon, and the subsequent acts cf im- 

 pregnation and segmentation have been skilfully 

 blended with the fundamental assumption in such 

 a way that they are made to seem to be a proof of 

 it. This strong position is now assailed by Dr. Oscar 

 Hertwig, who is in many respects peculiarly well fitted 

 to the task. He is the master of a simple, lucid and 

 logical style, he has himself been, in conjunction with 

 his brother, a pioneer in many of the discoveries on 

 which the doctrine of predetermination is founded, and 

 he has recently set himself to the task of verifying the 

 experiments of Roux and others, and of examining the 

 evidence which they afford for or against the doctrine 

 ivhich he attacks. His answer is unequivocal. The 

 phenomena of development are to be explained on epi- 

 .; enetic, not on evolutionary grounds, and the latter 

 n pothesis is contradicted by a number of well-ascer- 

 ained facts. 

 In an introductory chapter Dr. Hertwig refers to and 

 f pts Roux's definition of development, evolution, and 

 genesis, which may be repeated here, as they give 

 ision to terms which are often loosely used or little 

 erstood. By "development" is meant the origin 

 perceptible heterogeneity. Epigenesis means, not 

 crely the formal increase of perceptible heterogeneity 

 I a substance apparently similar but possibly extremely 

 iimplex, but a real increase of pre-existing heterogeneity, 

 evolution means, the becoming perceptible of pre- 

 ;xisting latent imperceptible differences. Dr. Hertwig 

 oes on to describe the positions taken up by Weismann 

 nd Roux, and deals particularly with the former, who, as 

 e shows, regards the germ as a veritable microcosm, in 

 Inch every separate variable part which appears in the 

 nirse of the whole ontogeny is represented by a living 

 irticle ; on the characters of these particles the 

 liaracters of the parts of the adult organism, whether 

 jomposed of one cell or many, depend. The sum of 

 Mese particles forms the germ-plasm. Agreeing with 

 ■ismann that a theory of heredity must be founded 

 ind brought into harmony with the cell theory (and 

 perefore rejecting the opposite view of Nageli), Dr. 

 Ilertwig proceeds to attack Weismann's fundamental 



NO. 1316, VOL. 51] 



assumptions. As he rightly says, the foundation and 

 corner-stone of Weismann's theory is the assumption of 

 differential or anisocleronomic' division of the cell 

 nucleus. All-important as this assumption is, there is 

 no foundation of fact to be found for it in all Weismann's 

 work. Instead we find purely dialectical argument, and 

 more than this, we find that Weismann has attributed the 

 most opposite characters to his '■ idioplasm," declaring 

 it, in one place, to be stable and unchangeable, in an- 

 other to be labile and changeable. But, Hertwig points 

 out, the facts are directly opposed to the assumption of 

 anisocleronomic division of the germ substance. In the 

 Protozoa the division is clearly isocleronomic, and we 

 know of no instance among them in which the act of divi- 

 sion, as such, is a means of producing new species. 

 Moreover, the numerous cases {e.g. Podophrya gemmi- 

 para) of complicated life histories among Protoza show 

 that the dissimilarity which may at first obtain between 

 the two products of cell division is no indication of per- 

 manent and essential difference. The case of the lowest 

 multicellular organisms is adduced as showing that in 

 these also the cell division is isocleronomic, for each cell 

 of which the soma of many of these organisms is com- 

 posed, retains the power of giving rise to the whole 

 organism. The phenomena of regeneration and hetero- 

 morphosis afford evidence that there are in the tissues of 

 many highly differentiated organisms cells, or groups 

 of cells, which retain, in a high degree, the power of 

 giving rise to new and complicated structures, and this 

 is particularly exemplified in cases of heteromorphosis, 

 which is to be distinguished from regeneration by the 

 fact that, in the former case, lost organs are replaced by 

 organs which differ in form and function from those 

 which were lost, or organs are, as a result of special 

 conditions, produced in abnormal positions on the body. 

 Under this head of phenomena of heteromorphosis, Dr. 

 Hertwig groups the extraordinary phenomena which 

 have been brought to light by his own researches and 

 those of Driesch and E. B. Wilson on the segmenting 

 ova of such different animals as frogs, echinids, and 

 amphioxus, and these observations have been extended, 

 since the publication of Dr. Hertwig's book, by the re- 

 searches of Prof Rafaello Zoja on the developing ova 

 of ccelenteratcs. Briefly stated, the results of these 

 experiments are as follows : In the case of echinids, 

 amphioxus, and ccclenterates, the first two, four, or even 

 eight blastomeres may, by suitable means, be isolated 

 without impairing their vitality. Each blastomere, in- 

 stead of giving rise to an incomplete embryo, or a portion 

 of an embryo, begins the developmental course afresh, 

 as it were, and produces an embryo perfect in all its 

 parts, but one-half, one-fourth, or one-eighth the size, as 

 the case may be, of the normal embryo. In the case of 

 the frog it was not possible to isolate the blastomeres ; 

 but Hertwig was able, by pressure, to so alter the seg- 

 mentation, that the normal relations of the blastomeres, 

 one to another, were completely changed, and yet a 

 perfectly normal embryo resulted. Unquestionably this 

 proves that, in the first stages of segmentation, at any 

 rate, the division of the germ substance is not differen- 



1 The German words " elbplcich " and " erbunglcich " being untrans- 

 latable into English, I have coined the equivalents isocleronomic ODd aniso- 

 cleronomic from the Greek KA7)pdi'Ofios an heir. — G. C H. 



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