22 LIFE: ITS NATUEE, ORIGIN AND MAINTENANCE 



employment of the microscope, that we and all the 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 constitu- 

 tion 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 ! 

 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 phenomena 

 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. 



Let us consider how cell-aggregates came to be evolved from organisms 

 consisting of single cells. Two methods are possible viz. : (1) The 



adhesion of a number of originally separate indi- 

 viduals ; ( 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 continuity : sub- 

 sequently a space or cavity became formed in the interior of the mass, 

 which was thus converted into a hollow sphere. All the cells of the aggre- 

 gate were at first perfectly similar in structure and in function ; there was 

 no subdivision of labour. All 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 cor- 

 respondingly altered in shape. With this change in structure differentia- 

 tion of function between the cells covering the outside and those lining the 

 inside of the cup made its appearance. Those on the outside subserved 

 locomotor functions and received and transmitted from cell to cell stimuli, 

 physical or chemical, received by the organism ; while those on the inside, 

 being freed from such functions, tended to specialise in the direction of 

 the inception and digestion of nutrient material; which, passing from 

 them into the cavity of the invaginated sphere, served for the nourish- 



