258 THE EVOLUTION THEORY 



spherical body, a whole, by a gelatinous matrix which they all secrete, 

 and thus they form a cell-colony, a cell-stock, a many -celled individual ; 

 but each of these cells has not only all the typical parts — cell-body, 

 nucleus, and contractile vacuole — but each possesses a pair of flagella or 

 motor organs, an eye-spot, and a chlorophyll body which enables them 

 to assimilate nourishment from the water and the air. Each one of 

 these cells thus performs all the somatic functions, that is, all that are 

 necessary to the maintenance of the individual life. But each also 

 possesses the power of reproducing the whole colony from itself, that 

 is, it also performs the function of reproduction necessary to the 

 maintenance of the species. When such a colony, whose sixteen cells 

 are continually growing, has led for some time a free-swimming life 

 in the water, the cells retract their flagella, and each begins to 

 multiply by dividing into a, 4, 8, finally into 16 cells of the same 

 kind, which remain together, forming a spherical mass enclosed in a 

 gelatinous secretion (Fig. 62, II). Thus there are now, instead of 

 sixteen cells in the mother-colony, sixteen daughter-colonies, each 

 with sixteen cells which soon acquire flagella and eye-spots, and are 

 then ready to burst forth from the dissolving jelly of the maternal 

 stock as independent individuals. This Pandorina shows no trace of 

 a differentiation of its component cells to particular and different 

 functions, but a nearly allied genus of the same family, the genus 

 Volvox (Fig. 62, III), consists of two kinds of cells — on the one hand 

 of small cells {sz) which occur in large numbers and compose the wall 

 of the hollow gelatinous mass, forming, so to speak, the skeleton of 

 the Volvox ; and, on the other hand, of a much smaller number of 

 cells which are very much larger (Jiz). The former, the ' body ' or 

 ' somatic ' cells, are green, and have a red ' eye-spot ' and two flagella ; 

 they are connected with each other by processes from their cell-bodies, 

 and are able, by means of the co-ordinated action of their flagella, to 

 propel the whole colony with a slow rotatory movement through 

 the water. Many of my readers are doubtless familiar with these 

 light green spheres, which are quite recognizable with the naked eye, 

 and people our marsh pools and ponds in Spring in such abundance 

 that it is only necessary to draw a glass of water to procure a large 

 number of them. 



The little flagellated cells just described serve not only for the 

 locomotion of the colony, but also for nutrition', for the secretion of 

 the jelly, and for the excretion of waste products; in short, they 

 perform all the functions necessary to the maintenance of life, but not 

 that of reproduction. They can, indeed, multiply by dividing when 

 the colony is young, like the cells of Pandorina, but they cannot 



