September 5, 1912] 



NATURE 



13 



tion of effecting profound chemical changes in other 

 material in contact with it without itself undergoing 

 permanent change. This catalytic function may have 

 been exercised directly by the living substance or may 

 have been carried on through the agency of the 

 enzymes already mentioned, which are also of a colloid 

 nature but of simpler constitution than itself, and 

 which differ from the catalytic agents employed 

 by the chemist in the fact that they produce their 

 effects at a relatively low temperature. In the course 

 of evolution special enzymes would become developed 

 for adaptation to special conditions of life, and with 

 the appearance of these and other modifications, a 

 process of differentiation of primitive living matter 

 into individuals with definite specific characters 

 gradually became established. We can conceive of 

 the production in this way from originally undif- 

 ferentiated living substance of simple differentiated 

 organisms comparable to the lowest forms of Protista. 

 But how long it may have taken to arrive at this 

 stage we have no means of ascertaining. To judge 

 from the evidence afforded by the evolution of higher 

 organisms it would seem that a vast period of time 

 would be necessary for even this amount of organisa- 

 tion to establish itself. 



Formation of the Nucleated Cell. 



The next important phase in the process of evolu- 

 tion would be the segregation and moulding of the 

 diffused or irregularly aggregated nuclear matter into 

 a definite nucleus around which all the chemical 

 activity of the organism will in future be centred. 

 Whether this change were due to a slow and gradual 

 process of segregation or of the nature of a jump, 

 such as Nature does occasionally make, the result 

 would be the advancement of the living organism to 

 the condition of a complete nucleated cell : a material 

 ad\-ance not only in organisation, but — still more 

 important — in potentiality for future development. 

 Life is now embodied in the cell, and every living 

 being evolved from this will itself be either a cell or a 

 cell-aggregate. Omnis cellula e ccllidd. 



Establishment uj Sexual Differences. 



After the appearance of a nucleus — but how long 

 after it is impossible to conjecture — another pheno- 

 menon appeared upon the scene in the occasional 

 exchange of nuclear substance between cells. In this 

 manner became established the process of sexual re- 

 production. Such exchange in the unicellular Protista 

 might and may occur between any two cells forming 

 the species, but in the multicellular Metazoa it became 

 — like other functions — specialised in particular cells. 

 The result of the exchange is rejuvenescence; asso- 

 ciated with an increased tendency to subdivide and 

 to produce new individuals. This is due to the intro- 

 duction of a stimulating or catalytic chemical agent 

 into the cell which is to be rejuvenated, as is proved 

 by the experiments of Loeb already alluded to. It is 

 true that the chemical material introduced into the 

 germ-cell in the ordinary process of its fertilisation 

 by the sperm-cell is usually accompanied by the intro- 

 duction of definite morphological elements which 

 blend with others already contained within the germ- 

 cell, and it is believed that the transmission of such 

 morphological elements of the parental nuclei is related 

 to the transmission of parental qualities. But we must 

 not be blind to the possibility that these transmitted 

 qualities may be connected with specific chemical 

 characters of the transmitted elements ; in other 

 words, that heredity also is one of the questions the 

 eventual solution of which we must look to the chemist 

 to provide. 



NO. 2236, VOL. go] 



.Aggregate Life. 



So far we have been chiefly considering life as it 

 is found in the simplest forms of living substance, 

 organisms for the most part entirely microscopic and 

 neither distinctively animal nor vegetable, which were 

 grouped together by Haeckel as a separate kingdom 

 of animated nature — that of Protista. But persons un- 

 familiar with the microscope are not in the habit of 

 associating the term "life" with microscopic 

 organisms, whether these take the form of cells or 

 of minute portions of living substance which have not 

 yet attained to that dignity. We most of us speak 

 and think of life as it occurs in ourselves and other 

 animals with which we are familiar ; and as we find 

 it in the plants around us. We recognise it in these 

 by the possession of certain properties — movement, 

 nutrition, growth, and reproduction. We are not 

 aware by intuition, nor can we ascertain without the 

 employment of the microscope, that we and all th« 

 higher living beings, whether animal or vegetable, 

 are entirely formed of aggregates of nucleated cells, 

 each microscopic and each possessing its own life. 

 Nor could we suspect by intuition that what we term 

 our life is not a single indivisible property, capable 

 of being blown out with a puff like the flame of a 

 candle ; but is the aggregate of the lives of many 

 millions of living cells of which the body is composed. 

 It is but a short while ago that this cell-constitution 

 was discovered : it occurred within the lifetime, even 

 within the memory, of some who are still with us. 

 What a marvellous distance we have travelled since 

 then in the path of knowledge of living organisms 1 

 The strides which were made in the advance of the 

 mechanical sciences during the nineteenth century, 

 which is generally considered to mark that century 

 as an age of unexampled progress, are as nothing 

 in comparison with those made in the domain of 

 biology, and their interest is entirely dwarfed by that 

 which is aroused by the facts relating to the pheno- 

 mena of life which have accumulated within the same 

 period. .And not the least remarkable of these facts 

 is the discovery of the cell-structure of plants and 

 animals ! 



Evohiiion of the Cell-aggregate. 



Let us consider how cell-aggregates came to be 

 evolved from organisms consisting of single cells. 

 Two methods are possible — viz. (i) the adhesion of a 

 number of originally separate individuals; (2) the 

 subdivision of a single individual without the products 

 of its subdivision breaking loose from one another. 

 No doubt this last is the manner whereby the cell- 

 aggregate was originally formed, since it is that by 

 which it is still produced, and we know that the life- 

 history of the individual is an epitome of that of the 

 species. Such aggregates were in the beginning solid ; 

 the cells in contact with one another and even in con- 

 tinuity : subsequently a space or cavity became 

 formed in the interior of the mass, which was thus 

 converted into a hollow sphere. .Ml the cells of the 

 aggregate were at first perfectly similar in structure 

 and in function ; there was no subdivision of labour. 

 .\11 would take part in effecting locomotion ; all would 

 receive stimuli from outside ; all would take 

 in and digest nutrient matter, which would then 

 be passed into the cavity of the sphere to serve as a 

 common store of nourishment. Such organisms are 

 still found, and constitute the lowest types of Metazoa. 

 Later one part of the hollow sphere became dimpled 

 to form a cup ; the cavity of the sphere became corre- 

 spondingly altered in shape. With this change in 

 structure, differentiation of function between the cells 

 covering the outside and those lining the inside of the 

 cup made its appearance. Those on the outside sub- 



