The Story of Evolution 67 



(b) Others were very sluggish, the parasitic Sporozoa, like the 

 malaria organism which the mosquito introduces into man's body. 



(c) Others were neither very active nor very passive, the 

 Rhizopods, with out-flowing processes of living matter. This 

 amoeboid line of evolution has been very successful; it is repre- 

 sented by the Rhizopods, such as Amoebae and the chalk-forming 

 Forminifera and the exquisitely beautiful flint-shelled Radio- 

 larians of the open sea. They have their counterparts in the 

 amoeboid cells of most multicellular animals, such as the 

 phagocytes which migrate about in the body, engulfing and 

 digesting intruding bacteria, serving as sappers and miners when 

 something has to be broken down and built up again, and per- 

 forming other useful offices. 



3 



The Making of a Body 



The great naturalist Louis Agassiz once said that the biggest 

 gulf in Organic Nature was that between the unicellular and the 

 multicellular animals (Protozoa and Metazoa). But the gulf 

 was bridged very long ago when sponges, stinging animals, and 

 simple worms were evolved, and showed, for the first time, a 

 ''body." What would one not give to be able to account for the 

 making of a body, one of the great steps in evolution! No one 

 knows, but the problem is not altogether obscure. 



When an ordinary Protozoon or one-celled animal divides 

 into two or more, which is its way of multiplying, the daughter- 

 units thus formed float apart and live independent lives. But 

 there are a few Protozoa in which the daughter-units are not 

 quite separated off from one another, but remain coherent. Thus 

 Volvox, a beautiful green ball, found in some canals and the like, 

 is a colony of a thousand or even ten thousand cells. It has 

 almost formed a body! But in this "colony-making" Protozoon, 

 and in others Jike it, the component cells are all of one kind, 

 whereas in true multicellular animals there are different kinds of 





