Feb. 6, 1890] 



NATURE 



319 



of the differentiation of homoplastids into the lowest 

 heteroplastids ; in Pandorina the cells are still all alike 

 and all perform the same functions, in Volvo x occur 

 somatic and germinal cells, and in the latter case we 

 should expect to find the commencement of natural death. 

 Recent researches of Dr, Klein (" Morphologische und 

 biologische Studien iiber die Gattung Volvox," Jahrb. 

 wiss. Botan., xx., 1889) show that this is actually the 

 case ; as soon as the germ-cells are ripe and emerge 

 from the sphere, the ciliated somatic cells begin to shrivel 

 up, and die in one or two days. This is the more interest- 

 ing, as the somatic are also the nutritive cells ; for, though 

 the germ-cells also possess chlorophyll, the rapid growth 

 of the latter (which attain an enormous size in Volvox) is 

 only possible by the supply of nourishment from the 

 somatic cells. The latter are so constituted that they 

 assimilate, but cannot grow larger when once the sphere 

 has reached its definite size ; they transfer the nourishment 

 which they derive from the decomposition of carbon 

 dioxide, &c., to the germinal cells by means of fine 

 pseudopodia ; and themselves wither when once the 

 germs are ripe. In this case adaptation to the nutrition 

 of the germinal cells might well have accelerated the 

 introduction of a natural death of the somatic cells, the 

 capacity for considerable assimilation combined with a 

 drain on their nutrition may have led after a certain 

 time to stoppage of the process of assimilation and to 

 death. To me, the idea that modification of the living 

 matter may have been connected with loss of immortality 

 does not appear more unlikely or more difficult than the 

 generally received view of the gradual differentiation of 

 the somatic cells in the course of phylogeny into their 

 various species of digestive, secretive, motile, and nervous 

 cells. An immortal unalterable living substance does not 

 exist, but only immortal forms of activity of organized 

 matter. 



I maintain, therefore, in its entirety, my original state- 

 ment, that monoplastids and the germ-cells of higher 

 forms have no natural death. I do not know how this 

 can to-day be better expressed than by saying that these 

 living units possess a real and actual immortality as 

 against the imaginary ideal immortality of the Greek 

 gods. If death from internal causes does not exist for 

 them, one may yet say with certainty that the fatal hour 

 will one day strike for them all, not from internal causes, 

 but because the external conditions for the constant 

 renewal of vital activity will some day cease. The 

 physicists prophesy that the circulation of water on the 

 globe will end, not from any alteration in the qualities of 

 water, but because external conditions will render this 

 form of motion of aqueous particles impossible. 



Prof Vines then attacks my view of embryogeny. He 

 finds it "not a little remarkable that Prof. \Veismann 

 should not have offered any suggestion as to the concep- 

 tion which he has formed of the mode in which the con- 

 version of germ-plasm into somatoplasm can take place, 

 considering that this assumption is the key to his whole 

 position." He sees here the same difficulty as in the 

 phyletic development, and says : " There is really no 

 other criticism to be made on an unsupported assumption 

 such as this, than to say that it involves a contradiction 

 in terms." He means by this that the eternal cannot 

 pass into the finite, as must be the case if the 

 immortal germ-cell grow into the mortal soma. At 

 the bottom of this objection lies the same confusion 

 between immortality and eternity which has already been 

 made clear. I do not wish to reproach Prof. Vines with 

 this obscurity, as I felt the same objection myself for 

 many years, and could not at once discover the reply to 

 it ; on the contrary, I am indebted to him for the oppor- 

 tunity to express myself on the point. Up to this time 

 we have had no scientific conception of immortality ; if 

 this be accepted, the significance of immortality is not 

 life without beginning or end, but life which, after its first 



commencement, can continue in'.efinitely with or without 

 modification (specific changes in the germ-plasm or the 

 monoplastids) ; it is a cyclical activity of organic material 

 devoid of any intrinsic momentum which would lead to 

 its cessation, just as the motion of the planets contains no 

 intrinsic momentum which would lead to its cessation, 

 although it has had a commencement and will some day, 

 through the operation of extrinsic forces, have an end. 



Prof. Vines says later : " I understand Prof. Weismann 

 to imply that his theory of heredity is not — like, for 

 instance, Darwin's theory of pangenesis — a provisional 

 or purely formal solution of the question, but one which 

 is applicable to every detail of embryogeny, as well as to 

 the more general phenomena of heredity and variation." 

 I have, as a matter of fact, designated Darwin's pangenesis 

 as a " purely formal " solution of the question, but should 

 like here to give a slight explanation of the expression, as 

 I fear that not only Prof. Vines, but also many other 

 readers of my essays, have misunderstood me. On the 

 one hand, I am afraid that they see in my words a definite 

 reproach against Darwin for his theory of pangenesis, of 

 which I had not the remotest intention ; and on the other, 

 that they incline to charge me with too great an affection 

 for my own theory. 



I believe there are two kinds of theory ; one may term 

 them the " real " and the " ideal " ; practically they are 

 rarely sharply to be discriminated ; both often occur in 

 one and the same theory, but should be conceived of 

 separately. The " ideal " theories attempt to render con- 

 ceivable the phenomena to be explained by an arbitrarily 

 accepted principle, apart from the question whether the 

 principle itself possesses any grain of truth or not ; they 

 seek only to show that there are hypotheses on which the 

 phenomena in question become comprehensible. " Real " 

 theories do not make hypotheses at pleasure, but strive 

 to construct such as have some degree of probability ; 

 they desire to give not a formal, but, if possible, the right 

 explanation. Sir William Thomson in endeavouring to 

 make clear the dispersion of rays of light, never believed in 

 the remotest degree that such molecules as he pictured 

 really existed, but desired merely to show that there were 

 hypotheses on which the phenomena of dispersion were 

 comprehensible. Darwin's pangenesis was originally in- 

 tended in this sense, and was by him termed a " pro- 

 visional" hypothesis, although in later years he may have 

 attributed to it the weight of a real theory. To me his "gem- 

 mules" are a pure invention, an invention in no way corre- 

 sponding to the actual facts, but showing what hypotheses 

 must be made in order to explain the phenomena of 

 heredity. Are, however, such ideal theories worthless } 

 Certainly not. They are often the first and essential step 

 towards the understanding of complicated phenomena, and 

 lay the foundation for the gradual erection of a real theory. 

 It would perhaps never have occurred to me to deny 

 the inheritance of acquired characters, had not Darwin's 

 pangenesis shown me that the matter was only explicable 

 on an hypothesis so difficult to conceive, as that of the 

 giving off, circulation, and reassemblage of gemmules. I 

 do not even now maintain that Darwin's pangenesis cannot 

 possibly contain a kernel of truth ; De Vries (" Intra- 

 cellulare Pangenesis," Jena, 1889) has shown in a recent 

 and most interesting memoir that the ideal impossible 

 pangenesis may be transformed into a real and possible 

 one by means of certain profound modifications ; he ac- 

 cepts my view that acquired (somatogenic) modifications 

 cannot be transmitted, and thereby puts on one side just 

 that part of Darwin's theory which has always appeared 

 to me to lie beyond the pale of reality — namely, the circula- 

 tion, &c., of the gemmules. The future will show whether 

 his view of modified gemmules or my hypothesis is the 

 best explanation of the facts of heredity. 



In any case, I am far from assuming that I have settled 

 the whole question of heredity ; I have undertaken re- 

 searches on some of the more important parts of the 



