84 



NATURE 



[September 15, 1921 



The Stream of Life.^ 

 By Prof. Arthur Dendy, F.R.S. 



ALL typical organisms — animal or vegetable — 

 are composed of cells; minute nucleated 

 masses of protoplasm, existing either singly or 

 in many-celled aggregates. These cells are 

 capable of reproducing themselves by a process 

 of division, and each of the higher organisms, 

 with certain negligible exceptions, starts its life in 

 the condition of a single cell which we call an 

 ^g'g' ^^ ovum, or, to use a more general term, 

 a germ-cell. 



Whatever may have happened in the far-distant 

 past, at the present day, so far as we can see, 

 every living thing is the product of some pre- 

 existing living thing, the relation of parent and 

 child holds good throughout the whole organic 

 world, and when we come to analyse this relation- 

 ship from the biological point of view we find that 

 it is always essentially based upon cell-division. 

 Leaving out of account, as we may legitimately do 

 for our present purposes, the stages of proto- 

 plasmic evolution that precede the appearance of 

 the nucleated cell, we may say that the cell is the 

 unit of organic structure, that all organisms are 

 built up of such units in somewhat the same way 

 as a house is built up of bricks, except that the 

 process of building in the living organism is one 

 of cell-growth and cell-multiplication, while the 

 bricks of a house are brought together and com- 

 bined into a building by some external agency. 

 This fundamental conception of organic growth 

 leads to the still more fundamental conception of 

 living matter as a continuous stream of proto- 

 plasm, starting with the first appearance of life 

 on the earth and continuing to the present day 

 with undiminished vigour; but it is a stream 

 which in the process of time constantly branches 

 out in new directions, giving rise ever to more 

 complex and more diversified types of plants and 

 animals. It is the stream of life. To make use 

 of a more familiar metaphor, the whole organic 

 world may be.compared to a great tree, the roots 

 of which are dead and buried in the past, and 

 the leaves and flowers of which, individualised 

 and endlessly diversified, are represented by the 

 living plants and animals of to-day. 



Let us now examine a little more closely the 

 means by which successive generations of 

 organisms, parents and offspring, are linked 

 together. We have to ask ourselves the question : 

 Why do all except the simplest organisms repro- 

 duce themselves by means of eggs, instead of 

 simply dividing up into equivalent parts? In 

 other words, why does every plant and animal 

 of a new generation have to go back to the begin- 

 ning and start its life as a single cell? We may 

 approach this question by considering for a 

 moment a once familiar domestic operation — the 

 baking of bread. You have no doubt sometimes 



1 From a citizent' lecture delivered at Edinburgh on September 8 during 

 the meeting of the British Association; 



NO. 2707, VOL. 108] 



heard the expression "half-baked" applied to 

 human beings, which shows that the analogy I 

 propose to make use of is not altogether new. 

 If we want to increase the number of our loaves 

 it is no good simply cutting them in halves. We 

 must go back to the dough and out of it fashion 

 new loaves. The dough, properly prepared, con- 

 tains all the ingredients necessary for bread- 

 making and is capable of developing into loaves 

 when subjected to the right treatment. When 

 once it has developed into a loaf, however, it 

 cannot be turned back again into dough. 



So it is also with the living organism. When 

 once the protoplasm of the ^g§, or germ-plasm, as 

 it is technically termed, has developed into the 

 mature tissues and organs of the adult body, it 

 cannot, usually at any rate, be turned back again 

 into germ-plasm ; it continues to live for a time, 

 but the stress and strain of life gradually exhaust 

 its vitality ; for a time, tissues and organs may 

 be renewed, but ultimately some essential part of 

 the mechanism of the body is worn out beyond 

 the possibility of repair, and the death of the 

 entire organism inevitably follows. 



What provision, then, is made for the next 

 generation — who mixes the next batch of dough? 

 Here I am afraid our analogy breaks down, and 

 it breaks down just because the germ-plasm, 

 unlike the dough, is a living substance capable of 

 increasing itself indefinitely by growth and multi- 

 plication. What happens is typically this — a part 

 of the original germ-plasm of each generation is 

 set aside, taking no share in the development of 

 the body, but remaining in the condition of com- 

 paratively undifferentiated protoplasm, while con- 

 tinuing to increase and subdivide into germ-cells. 

 It thus appears that the old idea that the hen 

 produces the eg^ is scarcely correct — it seems that 

 the e.gg produces the hen and at the same time 

 more eggs, which are accidentally, as it were, 

 included in the body of the hen.- The constant 

 succession of germ-cells, each produced by division 

 of a parent cell, constitutes the only really con- 

 tinuous stream of living protoplasm. The bodies 

 of individual plants and animals, developing from 

 the germ-cells, may be compared to local and 

 temporary overflows from the stream, which 

 sooner or later dry up and disappear, or, in other 

 words, die. This is Weismann's well-known doc- 

 trine of the "continuity of the germ-plasm," and 

 for our present purposes we may take it as sub- 

 stantially correct, in principle if not in detail. 



The simpler living organisms, which, like the 

 amoeba, consist each of only a single cell, are 

 exempt from death, because in them the stream 

 of protoplasm forms no overflows ; it consists 

 entirely of germ-plasm, and no differentiated 

 bodies are formed, so that there is nothing to 

 die, nothing which cannot go on reproducing itself 

 indefinitely. Death is the penalty paid for a higher 



