624 



NATURE 



{Oct. 24, 1889 



Still more is the theory open to criticism from the 

 standpoint which I have established above. It is clear 

 that the theory of the continuity of the germ-plasm, as 

 explaining heredity, is only valid on the assumption that 

 the germ-plasm of the ovum gives rise to the somato- 

 plasm of the embryo. But I have shown above that the 

 conversion of germ-plasm into somatoplasm is inconceiv- 

 able ; and, even if it be admitted, it cannot be seriously 

 maintained that the whole body of the embryo is, in any 

 case, developed solely from the germ-plasm of the ovum. 

 On the contrary, since the embryo is developed from the 

 whole of the nucleus and more or less of the cytoplasm 

 of the ovum, it must be admitted that the non-germ- 

 plasm, or somatoplasm, of the ovum provides a large 

 part of the material in embryogeny. It is an obvious 

 inference that, under these circumstances, hereditary 

 characters may be transmitted from the parent to the 

 offspring, not only by the germ-plasm, but also by the 

 somatoplasm, of the ovum. 



It might be replied to these criticisms that, even if it 

 be admitted that germ-plasm cannot be converted into 

 somatoplasm, and also that the somatic cells of the em- 

 bryo are derived from the somatoplasm of the ovum, it 

 is still conceivable that the nuclei of the somatic cells of 

 the embryo may contain a certain amount of the germ- 

 plasm of the ovum, not enough to confer upon the 

 somatic cells the properties of germ-cells, but sufficient 

 to determine their growth and differentiation in accord- 

 ance with the hereditary tendencies of which the germ- 

 plasm is the bearer. But this view does not appear to 

 be held by Prof Weismann, whose opinion with refer- 

 ence to the presence of germ-plasm in somatic cells is as 

 follows (p. 211) : — 



" I believe I have shown that theoretically hardly any objec- 

 tions can be raised against the view that the nuclear substance 

 of somatic cells may contain unchanged germ-plasm, or that this 

 germ-plasm may be transmitted alonij certain lines. It is true 

 that we might imagine a priori that all somatic nuclei contain a 

 small amount of unchanged germ-plasm. In Hydroids such an 

 assumption cannot be made, because only certain cells in a cer- 

 tain succession possess the power of developing into germ-cells ; 

 but it might well be imagined that in some organisms it would 

 be a great advantage if every part possessed the power of grow- 

 ing up into the whole organism, and of producing sexual cells 

 under appropriate circumstances. Such cases might exist if it 

 were possible for all somatic nuclei to contain a minute fraction 

 of unchanged germ-plasm." 



After alluding to the fact that new plants can be deve- 

 loped from leaves of Begonia which have been cut off 

 and laid in moist sand, Prof. Weismann continues : — 



" But I think that this fact only proves that, in Begonia and 

 similar plants, all the cells of the leaves, or perhaps only certain 

 cells, contain a small amount of germ-plasm, and that, conse- 

 quently, these plants are specially adapted for propagation by 

 leaves. How is it, then, that all plants cannot be reproduced in 

 this way ? No one has ever grown a tree from a leaf of the 

 lime or oak, or a flowering plant from the leaf of the tulip or 

 convolvulus. It is insufficient to reply that, in the last-men- 

 tioned cases, the leaves are more strongly specialized, and have 

 thus become unable to produce the germ substance ; for the 

 leaf-cells in these different plants have hardly undergone histo- 

 logical differentiation in different degrees. If, notwithstanding, 

 the one can produce a flowering plant, while the others have 

 not the power, it is of course clear that reasons other than the 

 degree of histological differentiation must exist ; and, according 

 to my opinion, such a reason is to be found in the admixture of 

 a minute quantity of unchanged germ-plasm with some of their 

 nuclei." 



It appears, therefore, to be Prof. Weismann's opinion 

 that it is only in special cases that germ-plasm is present 

 in somatic cells, and that, when present, it confers on 

 the somatic cells the properties of germ-cells, though it 

 is difficult to reconcile this opinion with the following 

 statement on p. 205 : — 



" Strictly speaking, I have therefore fallen into an inaccuracy 

 in maintaining (in former works) that the germ-cells are them- 

 selves immortal ; they only contain the undying part of the 

 organism — the germ-plasm ; and although this substance is, as 

 far as we know, invariably surrounded by a cell-body, it does 

 not always control the latter, and thus confer upon it the cha- 

 racter of a germ-cell." 



I would submit, therefore, that, inasmuch as Prof. 

 Weismann offers no evidence to prove the continuity of 

 the germ-plasm of the ovum with the somatoplasm of the 

 embryo, his principle of the continuity of the germ-plasm 

 cannot be regarded as a satisfactory theory of heredity ; 

 and I would point out that the facts of embryogeny seem 

 to confer upon the idea of a continuity of the somato- 

 plasm at least as high a degree of probability as upon 

 that of a continuity of the germ-plasm. 



We come, finally, to Prof. Weismann's explanation of 

 variation, a connected statement of which is to be foimd 

 on pp. Ill et seq., from which I may make the following 

 quotations :— 



"The origin of hereditary individual variability cannot indeed 

 be found in the higher organisms — the Metazoaand Metaphyta ; 

 but it is to be sought in the lowest — the unicellular organisms. 

 In these latter the distinction between body-cell and germ-cell 

 does not exist. Such organisms are reproduced by division, and 

 if, therefore, any one of them becomes changed in the course of 

 its life by some external influence, and thus receives an indi- 

 vidual character, the method of reproduction insures that the 

 acquired peculiarity will be transmitted to its descendants. If, 

 for instance, a Protozoon, by constantly struggling against the 

 influence of mechanical currents in water, were to gain a some- 

 what denser and more resistant protoplasm, or were to acquire 

 the power of adhering more strongly than other individuals of 

 its species, the peculiarity in question would be directly con- 

 tinued on into its two descendants, for the latter are at first nothing 

 more than the two halves of the former. It therefore follows 

 that every modification which appears in the course of its life, 

 every individual character, however it may have arisen, must 

 necessarily be directly transmitted to the two offspring of a 

 unicellular organism " (p. 277). 



"We are thus driven to the conclusion that the ultimate origin 

 of hereditary individual differences lies in the direct action of 

 external influences upon the organism. Hereditary variability 

 cannot, however, arise in this way at every stage of organic 

 development, as biologists have hitherto been inclined to believe. 

 It can only arise in the lowest unicellular organisms ; and when 

 once individual difference had been attained by these, it neces- 

 sarily passed over into the higher organisms when they first 

 appeared. Sexual reproduction coming into existence at the 

 same time, the hereditary differences were increased and 

 multiplied, and arranged in ever-changing combinations " (p. 

 279). 



"It is, however, obvious that sexual reproduction will readily 

 afford such combinations of acquired characters, for by its means 

 the most diverse features are continually united in the same 

 individual, and this seems to me to be one of its most important 

 results. 



"I do not know what meaning can be attributed to sexual 

 reproduction other than the creation of hereditary individual 

 characters to form the material upon which natural selection may 

 work" (p. 281). 



In the essay entitled " On the Number of Polar Bodies 

 and their Significance in Heredity," Prof. Weismann 

 explains his conception of the mode in which sexual 

 reproduction promotes variability, showing, with the 

 assistance of diagrams, how the nuclear germ-plasm of a 

 fertilized ovum contains germ-plasms derived from the 

 ancestors of both parents. 



The conception of the process of variation which the 

 preceding passages (as well as others) produce in the 

 mind of the reader is that unicellular organisms acquired, 

 during the period of their entirely asexual reproduction, a 

 number of individual differences ; and that, since the 

 appearance of sexual reproduction, these ancestral cha- 

 racters have been combined in an infinite number of 

 ways, leading to the evolution of all existing varieties of 



