NATURE 



[Feb. 6, 1890 



clearer light. With regard to the immortality which I 

 attribute both to the unicellular organisms and to the 

 germinal cells of the multicellular, if I understand Prof. 

 Vines aright, he does not attack the proposition itself, but 

 has simply overlooked the explanation in my book of the 

 way in which mortal organisms arose out of immortal in 

 process of phyletic development, a process which must have 

 taken place if the Protozoa have developed in the course of 

 the world's history into the higher Metazoa, — "the first 

 difficulty is to understand how the mortal heteroplastides 

 can have been evolved from the immortal monoplastides." 

 My explanation was simply that which appears to be the 

 true one for the origin of every higher differentiation — 

 namely, the division of the cell-mass of the Protozoan, on 

 the principle of the division of labour, into two dissimilar 

 halves, differing in substance, and consequently also in 

 function ; from the one cell which performed all functions 

 comes a group of several cells which distribute themselves 

 over the work. In my opinion, the first such differentia- 

 tion produced two sets of cells, the one the mortal cells 

 of the body proper, the other the immortal germ-cells. 

 Prof. Vines certainly believes in the principle of the divi- 

 sion of labour, and in the part that it has played in the 

 development of the organic world, as well as I ; but it 

 seems to him that this division of a unicellular being into 

 somatic and germinal cells is impossible, and that my 

 explanation of the process by dissimilar division is 

 inadequate, because it strikes him as " absurd to say that 

 an immortal substance can be converted into a mortal 

 substance." 



There certainly does seem to be a great difficulty in 

 this idea, but in reality it arises simply from a confusion 

 of two conceptions — immortality and eternity. That 

 the Protozoa and the germ-cells of Metazoa are in a certain 

 sense immortal seems to me an incontrovertible proposi- 

 tion. As soon as one has clearly realized that the division 

 of amonoplastidisinno way connected with the death of 

 one part, there can be no further question that we have 

 to do with individuals of indefinite duration ; but this in 

 no way implies that they possess an eternal duration ; on 

 the contrary, we imagine that they have all had a be- 

 ginning. The conception of eternity, however, extends 

 into the past as well as the future ; it is without beginning 

 or end, and does not affect the present question ; it is an 

 entirely artificial conception, and has no real and com- 

 prehensible existence ; to express it more accurately, 

 eternity is merely the negation of the conception of 

 transitoriness. Of the objects with which natural science 

 deals, none are eternal except the smallest particles of 

 matter and their forces, certainly not the thousandfold sem- 

 blances and combinations under which matter and force 

 meet us. As I have said years ago, the immortality of 

 unicellular organisms, and of the germ-cells of the multi- 

 cellular, is not absolute but potential ; it is not that they 

 must live for ever as did the gods of the ancient Greeks 

 — Ares received a " mortal " wound, and roared for 

 pain like to ten thousand bulls, but could not die ; they 

 can die — the greater number do in fact die — but a pro- 

 portion lives on which is of one and the same substance 

 with the others. Does not life, here as elsewhere, depend 

 on metabolism — that is to say, a constant change of 

 material 1 And what is it, then, which is immortal ? 

 Clearly not the substance, but only a definite form of 

 activity. The protoplasm of the unicellular animals is of 

 such chemical and molecular structure that the cycle of 

 material which constitutes life returns even to the same 

 point and can always begin anew, so long as the neces- 

 sary external conditions are forthcoming. It is like the 

 • circulation of water, which evaporates, gathers into 

 clouds, and falls as rain upon the earth, always to eva- 

 porate afresh. And as in the physical and chemical 

 properties of water there is no inherent cause for the 

 cessation of this cycle, so there is no clear reason in the 

 physical condition of unicellular organisms why the cycle 



of life, i.e. of division, growth by assimilation, and 

 repeated division, should ever end ; and this charac- 

 teristic it is which I have termed immortality. It is the 

 only true immortality to be found in Nature — a pure 

 biological conception, and one to be carefully dis- 

 tinguished from the eternity of dead, that is to say 

 unorganized, matter. 



If then this true immortality is but cyclical, and is con- 

 ditioned by the physical constitution of the protoplasm, 

 why is it inconceivable that this constitution should be, 

 under certain circumstances and to a certain extent, so 

 modified that the metaboHc activity no longer exactly 

 follows its own orbit, but after more or fewer revolutions 

 comes to a standstill and results in death ? All living 

 matter is variable ; why should not variations in the 

 protoplasm have also occurred which, while they fulfilled 

 certain functions of the individual economy better, caused 

 a metabolism which did not exactly repeat itself, i.e. 

 sooner or later came to a condition of rest .'' I admit that 

 I feel such a descent from immortahty into mortality far 

 less remarkable than the permanent retention of immor- 

 tality by the monoplastids and germ-cells. Small, indeed, 

 must be the variations in the complicated qualities of 

 living matter to bring in their train such a fall ; and very 

 sharply must the essentials of its constitution be retained, 

 for metabolism to take place so smoothly without creating 

 in itself an obstacle to its own continuance ! Even if we 

 cannot penetrate into the mysteries of this constitution, 

 still we may say that a rigorous and unceasing natural 

 selection is unremittingly active in maintaining it at such 

 an exact standard as to preserve its immortality; and 

 every lapse from this standard is punished by death. 



I believe that I have proved that organs no longer in 

 use become rudimentary, and must finally disappear 

 solely by " panmixie " ; not through the direct action of 

 disuse, but because natural selection no longer main- 

 tains their standard structure. What is true for an organ 

 is true also for its function, since the latter is but the 

 expression of the qualities of material parts, whether we 

 can directly perceive their relations or not. If, then, as 

 we saw, the immortality of monoplastids depends on the 

 fact that the incessant metabolism of their bodies is ever 

 returning exactly to its starting-point, and produces no 

 such modifications as would gradually obstruct the repe- 

 tition of the cycle, why should that quality of the livnig 

 matter which causes immortality — nay, how could it be 

 retained — when no longer necessary.? It is obvious that 

 it was no longer necessary in the somatic cells of the 

 heteroplastids. From the instant that natural selection 

 relaxed its watch on this quality of immortality began the 

 process of panmixia which led to its abolition. Prof. 

 Vines will ask. How can one conceive of this process ? 

 I answer, Quite easily. When once individuals arose 

 among monoplastids, in the protoplasm of which occurred 

 such variation in chemical and molecular constitution as 

 to result in a gradual check on the metabolic cycle, it 

 would happen that these individuals died ; a permanent 

 variety could not grow out of such variations. But if 

 there arose among heteroplastids individuals with a 

 similar differentiation of the somatic cells, the death of 

 these cells would not be detrimental to the species, since 

 its continuance is ensured by the imtnortal germ-cells. 

 Upon the differentiation into germinal and somatic cells,^ 

 natural selection was, speaking metaphorically, trained 

 to bear on immortality of the germ-cells, but on quite 

 other qualities in the somatic cells — on motility, irritabi- 

 lity, capacity for assimilation, &c. We do not know whether 

 the attainment of these qualities was accompanied by a 

 constitutional alteration which caused the loss of immor- 

 tality, but it is at least possible ; and, if true, the somatic 

 cells will have lost their immortality even more rapidly 

 than through the unaided action of panmixia. 



In the fourth essay of my book, I have cited the two 

 Volvocinean genera Pandorina and Volvox as examples 



