THE PRESIDENTS ADDRESS. 467 



amongst themselves for the fulfilment of different functions, and 

 all co-operate in a common life which is fuller and more varied 

 in accordance with the greater complexity of structure. This 

 differentiation and division of labour amongst the constituent 

 cells of the multicellular body has undoubtedly been one of the 

 chief means by which progressive evolution has been rendered 

 possible. From the point of view of the individual, however, it 

 has its drawbacks. Each cell is no longer self-sufficing, it can 

 no longer perform, by itself all the functions necessary for con- 

 tinued existence. A muscle cell, for example, is dependent upon 

 the blood for its supply of food and oxygen, upon the nervous 

 system for its means of communication with other parts of the 

 body, and upon the skin for its protection. It can do its own 

 particular job remarkably well, but only by sacrificing the power 

 to do other things that are very necessary for its own existence. 

 It has ceased to live as an independent individual and has become 

 a mere constituent part, an organ, of an individual of a higher 

 order. Moreover, it sooner or later loses the power of reproducing 

 itself by multiplication, becomes worn out and dies. So it is with 

 the vast majority of all the cells of which the multicellular body 

 is composed. They become worn out and die, and the body as a 

 whole perishes, death being the inevitable price paid for progress. 

 Certain cells in the body, however, escape the general debacle. 

 These are the germ cells, and the reason for their exemp- 

 tion seems to lie in the fact that they never become highly 

 specialised, never exhaust themselves by work, and never lose the 

 power of multiplication. They survive and start the game afresh. 

 They have been aptly compared to so many unicellular Protozoa 

 enclosed within the multicellular body in it but not of it and, 

 like the Protozoa, enjoying at least a potential immortality. 



If we inquire how the multicellular condition arose from the 

 unicellular in the course of evolution we find an answer in two 

 directions. In the first place the existence of protozoon colonies 

 especially such forms as Yolvox shows us clearly the first step in 

 the transition, and in the second place we see the actual process 

 repeated with more or less accuracy in the development of all the 

 higher animals from the unicellular egg. The division of the egg 

 into embryonic cells or blastomeres in the process of segmentation 

 is exactly comparable with the multiplication of an amoeba by simple 

 fission. There is only one difference, and that is that the daughter 



