394 CONCLUSION 



all parts of the body where it is actually assimilated/ 

 A further consequence of the habit of living on soli^ 

 organic food is that most animals — at least most of 

 the higher animals — ^have to seek it, since it is vqiry 

 unevenly distributed through their surroundings, and 

 thus have to move actively about, both for this purpose 

 and in order to escape from being themselves devoured. 

 They must move in definite directions and capture 

 and devouf their prey in definite ways. Therefore 

 they must be sensitive to a great variety of external 

 impressions of definite kinds, and hence the gradual 

 development of the muscular system, the nervous 

 system and the sense organs. 



The higher animals are, in fact, brought into relation 

 through their specific life-needs with a much greater 

 range of their environment than is the case with plants, 

 and this fact has called forth the very high differentiation 

 and specialisation of their bodies. The - necessities of 

 locomotion have kept the bodies of active animals 

 compact, with the different specialised organs and tissues 

 very closely dependent on one another, so that no con- 

 siderable part can be detached without crippling or 

 killing the whole. This characteristic is known as integ- 

 ration (the making of a whole individual, the parts of 

 which are closely interdependent) and is a feature of the 

 highest (vertebrate) animals as well as of such highly 

 developed specialised invertebrates as the insects and 

 Crustacea. Integration becomes greater and greater as 

 we ascend the animal scale. The lower animals are 

 much less integrated — the worms for instance, which 

 can be cut into pieces without being killed — and when 

 we come down to such forms as the hydroid polyps we 

 find that they share many of the characters of plants. 

 They are fixed branching forms of indefinite continuous 



