576 THE WONDER OF LIFE 



tion was made from unicellular to multicellular animals. 



When certain simple organisms, unable fully to complete 

 that division into two or more separate units which normally 

 occurs at the limit of growth, began to form multicellular 

 ' bodies ' one of the greatest steps in evolution was 

 taken. It was perhaps with the acquisition of a 

 body that natural death began. It was certainly with 

 the acquisition of a body that there began the very advan- 

 tageous division of labour between body- cells and germ- 

 cells. Among the Protozoa there are often dimorphic 

 units which combine in fertilization or conjugation, and 

 they are the analogues of the ova and spermatozoa of higher 

 animals ; but it was only after the establishment of the 

 multicellular body that the sexes, in the strict sense, were 

 differentiated as sperm-producers and ovum-producers 

 males and females respectively. 



Another step with far-reaching consequences was the 

 replacement of the radial symmetry of polyp and jellyfish 

 by bilateral symmetry. It was some ' worm ' types which 

 began the useful habit of moving with one end of the 

 body always in front, and with this was associated the 

 acquisition of head-brains, the beginning of the process 

 which has led to our being able to tell our right hand from 

 our left. 



We think of what was implied in the discovery of an 

 oxygen-capturing respiratory pigment like haemoglobin, 

 or of an armouring substance like chitin which is char- 

 acteristic of the highly successful Arthropod alliance of 

 Crustaceans, Insects, Spiders and the like. The early 

 differentiations of striped muscle and specialized sense- 

 organs were other great events, and much was gained by 

 such a simple step as having a food- canal open posteriorly 



