LIFE AND DEATH. 14-7 



by the conditions of the external world, and this renders neces- 

 sary the process which we call adaptation. Thus it is just as in- 

 conceivable that either a homogeneous or a heterogeneous cell-colony 

 possessing- the physiological value of a multicellular individual 

 should continue to grow to an unlimited extent by continued cell- 

 division, as it is inconceivable that a unicellular being should 

 increase in size to an unlimited extent. In the latter case the 

 process of cell-division imposes a limit upon the size attained by 

 growth. In the former, the requirements of nutrition, respiration, 

 and movement must prescribe a limit to the growth of the cell- 

 colony which constitutes the individual of the higher species, just 

 as in the case of the unicellular Monoplastides, and it does not 

 affect the argument if we consider this limitation to be governed 

 by the process of natural selection. It would only be possible to 

 regulate the relations of the single cells of the colony to each other 

 by fixing the number of cells within narrow limits. During the 

 development of Magospfiaera one of the Homoplastides the cells 

 arrange themselves in the form of a hollow sphere, lying in a 

 gelatinous envelope. But the fact that reproduction does not follow 

 the simple unvarying rhythm of unicellular organisms is of more 

 importance ; for a rhythm of a higher order appears, in which each 

 cell of the colony separates from its neighbours, when it has 

 reached a certain size, and proceeds by very rapid successive 

 divisions to give rise to a certain number of parts which arrange 

 themselves as a new colony. The number of divisions is controlled 

 by the number of cells to which the colony is limited, and at first 

 this number may have been very small. With the introduction of 

 this secondary higher rhythm during reproduction, the first germ 

 of the Polyplastides became evident ; for then each process of fission 

 was not, as in unicellular organisms, equivalent to all the others ; 

 for in a colony of ten cells the first fission differs from the second, 

 third, or tenth, both in the size of the products of division and also 

 in remoteness from the end of the process. This secondary fission 

 is what we know as segmentation. 



It seems to me of little importance whether the first process of 

 segmentation takes place in the water or within a cyst, although it 

 is quite possible that the necessity for some protective structure 

 appeared at a very early period, in order to shield the segmenting 

 cell from danger. 



L 3 



