74 DIVERGENCE AND PARALLELISM 



much relative diversity amongst plants as 

 amongst animals, showing that differentiation 

 proceeds from within though it may be modified 

 from without. The methods adopted for securing 

 food determine in great measure the habits of 

 animals. Their instincts and intelligence are 

 concentrated upon their food-supply, reproduc- 

 tion being automatic and all besides incidental. 

 It is small wonder then that many animals 

 follow similar methods ; and in this way habitual 

 convergence is brought about. Thus the car- 

 nivorous, insectivorous, herbivorous, and fru- 

 givorous habits are widely disseminated ; and 

 community of habit and experience is often 

 accompanied by greater or less similarity of 

 structure of certain parts, giving rise to a two- 

 fold exhibition of convergence, bionomical and 

 homoplastic. 



Next to the function of nutrition we have to 

 consider the organs which perform that function, 

 i.e., the mechanism of nutrition. For the sake of 

 brevity we may concentrate our attention upon 

 the higher or leaf-bearing plants and the higher 

 or gut-bearing animals. Now some parasitic 

 plants, the so-called root-parasites (Balanophora, 

 Rafflesia), can manage without green leaves, 

 and some parasitic animals, both endoparasites 

 (Cestoda and Acanthocephala) and ectoparasites 

 (Sacculina), can dispense with a gut. When, 

 however, as in the great majority of vascular 



