DOMAIN OF EVOLUTIONAKY HYPOTHESIS 137 



At present we can only say so much : viz. that 

 those plant and animal groups (families) which not 

 only now but also during the whole of their ancient 

 existence have remained alike in definite characters 

 peculiar to themselves these we must regard as types. 



In this connection it is immaterial what rank they 

 are given in the present classification, whether they 

 form species, genus, family, order, class, or stock. Ginkgo 

 biloba is to-day a species, since all the individuals are 

 entirely alike ; but according to the above criteria it is 

 a type. That is recognized in systematic classification 

 itself, since it ranks Ginkgo biloba as a ' class ' in con- 

 sideration of its peculiarities in comparison with other 

 Gymnosperms. 



Hatteria forms a genus with several species, which, 

 however, differ but little ; its peculiarities, however, 

 raise its position to that of an order, so soon as it is 

 compared with other (in a wide sense) similar animals 

 (Reptilia), and so on. 



It is the object of palaeontology and biology to 

 determine the number of fundamental forms of animals 

 and plants. The task is difficult, since we know how 

 much, for instance, parasitism, adaptation to other 

 habitats, and transition to sessile modes of existence, 

 can influence, alter, increase, or decrease the habit and 

 the degree of differentiation. 



One thing certainly already appears now to be as 

 good as certain, viz. that at least some fundamental 

 forms in the animal and plant world are firmly retained ; 

 for this reason an attribution of all animals and all 



