October 14, 19 15] 



NATURE 



187 



turn by new secondary nuclei at certain crises in the 

 life-history. In view of these facts, it can be asserted 

 without hesitation that the ring of staining granules 

 in Arcella is composed of, or at least contains, true 

 chromatin-grains, extranuclear chromatin for which 

 R. Hertwig s term chromidia is now used universally. 

 Having now defined or explained, as well as I am 

 able, the terms of which I am about to make use, I 

 return to my main theme, the cell and its evolution. 

 To summarise the points already discussed, a typical 

 cell is a mass of protoplasm differentiated into two 

 principal parts or regions, the cytoplasm and the 

 nucleus, or, it may be, two or more nuclei. The 

 cytoplasm may or may not contain chromatin-grains 

 in addition to other enclosures, and may possess cell- 

 organs of various kinds. The nucleus, highly vari- 

 able in minute structure, possesses one invariable con- 

 stituent, the chromatin-material in the form of grains 

 and masses of various sizes. 



The cell, therefore, in its complete and typical form, 

 is an organism of very considerable complexity of 

 structure and multiplicity of parts. The truth of this 

 proposition is sufficiently obvious even from simple 

 inspection of the structural details revealed by the 

 microscope in cells in the so-called "resting condi- 

 tion," but still more so from a study of their activities 

 and functions. The vital processes exhibited by the 

 cell indicate a complexity of organisation and a 

 minuteness in the details of its mechanism which 

 transcend our comprehension and baffle the human 

 imagination, to the same extent as do the immensities 

 of the stellar universe. If such language seems 

 hyperbolic, it is but necessary to reflect on some of 

 the established discoveries of cytology, such as the 

 extraordinary degree of complication attained in the 

 process of division of the nucleus by karyokinesis, or 

 the bewildering series of events that take place in the 

 nuclei of germ-cells in the processes of maturation 

 and fertilisation. Such examples of cell-activity give 

 us, as it were, a glimpse into the workshop of life 

 and teach us that the subtlety and intricacy of the 

 cell-microcosm can scarcely be exaggerated. 



On the assumption that an organism so complex 

 and potent was not created suddenly, perfect and com- 

 plete as it stands, but arose, like all other organisms, 

 by progressive evolution and elaboration of some 

 simpler form and type of structure, it is legitimate 

 to inquire which of the various parts of the cell are 

 the older and more primitive and which are more 

 recent acquisitions in the course of evolution. The 

 evolution of the cell may be discussed as a morpho- 

 logical problem of the same order as that of the phylo- 

 geny of any other class or phylum of living beings, 

 and by the same methods of inquiry. 



The problem of cell-evolution may be attacked, 

 beginning with the consideration of the primary struc- 

 tural differentiation of the typical cell, the distinction 

 of nucleus, or rather chromatin, and cytoplasm. 



Since no concrete foundation can be* found for the 

 view that cytoplasm and chromatin have a common 

 origin in the evolution of living things, we are 

 brought back to the view that one of them must have 

 preceded the other in phylogeny. 



For my part, I am unable to accept any theory of 

 the evolution of the earliest forms of living beings 

 which assumes the existence of forms of life com- 

 posed entirely of cytoplasm without chromatin. All 

 the results of modern investigations into the structure, 

 physiology, and behaviour of cells, on the one hand, 

 and of the various types of organisms grouped under 

 the Protista, on the other hand — the combined results, 

 that is to say, of cytology and protistology — appear 

 to me to indicate that the chromatin-elements repre- 

 sent the primary and original living units or in- 

 dividuals, and that the cytoplasm represents a secon- 

 NO. 2398, VOL. 961 



dary product. I will summarise briefly the grounds 

 that have led me to this conviction, and will attempt 

 to justify the faith that 1 hold; but first I wish to 

 discuss briefly certain preliminary considerations 

 which seem to me of great importance in this con- 

 nection. 



It is common amongst biologists to speak of 

 "Uiving substance," this phrase being preceded by 

 either the definite or the indefinite article — by either 

 "the" or "a." If we pause to consider the meaning 

 of the phrase, it is to be presumed that those who 

 make use of it employ the term "substance" in the 

 usual sense to denote a form of matter to which some 

 specific chemical significance can be attached, which 

 could conceivably be defined more or less strictly by a 

 chemist, perhaps even reduced to a chemical formula 

 of some type. But the addition of the adjective 

 "living" negatives any such interpretation of the 

 term " substance," since it is the fundamental and 

 essential property of any living being that the 

 material of which it is composed is in a state of 

 continual molecular change and that its component 

 substance or substances are inconstant in molecular 

 constitution from moment to moment. When the 

 body of a living organism has passed into a state uf 

 fixity of substance, it has ceased, temporarily or per- 

 manently, to behave as a living body; its fires are 

 banked or extinguished. The phrase " living sub- 

 stance" savours, therefore, of a contradictio in ad- 

 jecto; if it is "living" it cannot be a "substance," 

 and if it is a " substance " it cannot be " living." 



As a matter of fact, the biologist, when dealing 

 with purely biological problems, knows nothing cf 

 a living substance or substances ; he is confronted 

 solely by living individuals, which constitute his 

 primary conceptions, and the terms "life" and 

 "living substance" are pure abstractions. Every 

 living being presents itself to us as a sharply limited 

 individual, distinct from other individuals and con- 

 stituting what may be termed briefly a microcosmic 

 unit, inasmuch as it is a unity which is far from 

 being uniform in substance or homogeneous in com- 

 position, but which, on the contrary, is characterised 

 by being made up of an almost infinite multiplicity 

 of heterogeneous and mutually interacting parts. 

 We recognise further that these living individuals 

 possess invariably specific characteristics ; two given 

 living individuals may be so much alike that we 

 regard them as of the same kind or " species," or 

 they may differ so sharply that we are forced to dis- 

 tinguish between them specifically. Living beings 

 are as much characterised by this peculiarity of 

 specific individuality as by any other property or 

 faculty which can be stated to be an attribute of life 

 in general, and this is true equally of the simplest 

 or the most complex organisms ; at least we know 

 of no form of life, however simple or minute, in 

 which the combined features of individuality and 

 specificity are not exhibited to the fullest extent. 



The essential and distinctive characteristic of a 

 living body of any kind whatsoever is that it ex- 

 hibits while it lives permanence and continuity of in- 

 dividuality or personality, as manifested in specific 

 behaviour, combined with incessant change and 

 lability of substance; and further, that in reproducing 

 its kind, it transmits its specific characteristics, with, 

 however, that tendency to variability which permits 

 of progressive adaptation and gradual evolutionary 

 change. It is the distinctively vital property of 

 specific individuality combined with "stuff-change" 

 (if I may be allowed to paraphrase a Teutonic idiom) 

 which marks the dividing line between biochemistry 

 and biology. The former science deals with sub- 

 stances which can be separated from living bodies, 

 and for the chemist specific properties are associated 



