290 THE PHILOSOPHY OF BIOLOGY 



species and the higher groupings are therefore conve- 

 nient ways of symbolising the results of morphological 

 and physiological investigations, although they also 

 indicate the main directions taken by the evolutionary 

 process, but the manner in which they are stated in 

 taxonomic schemes is always a more or less formal one. 

 There are no absolute distinctions between group 

 and group, even between the animals and the plants. 

 There is nothing, for instance, in the morphology of a 

 Diatom to indicate that it belongs to the vegetable 

 kingdom, or in that of a Radiolarian, to indicate that 

 it is an animal. Peridinians are either plants or animals 

 according to the general argument, or the point of 

 view of the author who writes about them. Even a 

 study of the energy-transformations that are effected 

 in the living substance of these lower organisms does 

 not afford an absolute distinction : synthetic metabolic 

 processes in which energy passes into the potential 

 condition may be carried out in animals, while many 

 plants — the saprophytic fungi, or the insectivorous 

 plants, for instances — may effect analytic energy- 

 transformations of essentially the same nature as those 

 exhibited in the typical mode of animal metabolism. 

 Motility and the possession of a sensori-motor system 

 do not afford the means of making a sharply drawn 

 line of division between plants and animals. Potential 

 energy passes into the condition of kinetic energy in 

 the typical animal, and this kinetic energy is directed 

 by the sensori-motor system. But some lower uni- 

 cellular plants are motile, and they possess the rudi- 

 ments of a sensori-motor system in the flagella by which 

 their movements are effected. On the other hand, the 

 sensori-motor system has become vestigial in many 

 animal parasites — in the Crustacean Sacculina, for in- 

 stance, which is parasitic on some Crabs. The possession 



