2 CLASSIFICATION AND ADAPTATION 



plants is based on a different mental conception 

 of them : it regards them primarily as living active 

 organisms, not as dead and preserved specimens, 

 and it can only be carried on successfully by observ- 

 ing them in their natural conditions, in the wide 

 spaces of nature, under the open sky. 



The object of this kind of inquiry is to ascertain 

 what are the uses of organs or structures, what they 

 are for, as we say in colloquial language, to discover 

 what are their functions and how these functions 

 are useful or necessary to the life of the animals or 

 plants to which they belong. For example, some 

 Cuttle-fishes or Cephalopoda have eight arms or 

 tentacles and others ten. The taxonomist notices 

 the fact and distinguishes the two groups of 

 Octopoda and Decapoda. 



But it is also of interest to ascertain what is the 

 use of the two additional arms in the Decapoda. 

 They differ from the other arms in being much 

 longer, and provided with sockets into which they 

 can be retracted, and suckers on them are limited 

 to the terminal region. In the majority of zoo- 

 logical books in which Cephalopoda are described, 

 nothing is said of the use or function of these two 

 special arms. Observation of the living animal 

 in aquaria has shown that their function is to 

 capture active prey such as prawns. They act as 

 a kind of double lasso. Sepia, for instance, ap- 

 proaches gently and cautiously till it is within 

 striking distance of a prawn, then the two long ten- 

 tacles are suddenly and swiftly shot out from their 

 sockets and the prawn is caught between the suckers 

 at the ends of them. Another example is afforded 

 by the masked crab {Corystes cassivelaunus). This 



