CHAPTER I 



INTRODUCTORY 



No one, even though ignorant of Biology, can fail 

 to recognise that living things may be arranged 

 under one or other of the two categories, known 

 familiarly as plants and animals ; but although it is Plants 

 comparatively easy to appreciate the differences 

 between an oak tree and a horse, and to recognise 

 that these differences are sufficiently great to justify 



FIG. 1. A coral (A) and a coralline seaweed (B) 

 (J natural size.) 



us in keeping such organisms far apart in any scheme 

 of classification we may adopt, it is by no means so 

 easy to say offhand what characters they possess in 

 common. We recognise that they are both alive, 

 but, without some expert knowledge, we are likely 

 to find ourselves nonplussed if asked to say why we 

 regard them both as alive. 



\Vhen we come to consider organisms obviously 

 l A 



