ANIMAL BEHAVIOUR 185 



mal, and the modifications are regulatory, not haphazard, 

 in character. The phenomena are thus similar to those shown 

 in the ' learning ' of higher organisms, save that the modifica- 

 tions depend upon less complex relations and last a shorter 

 time" (p. 179). Our view of living creatures must make 

 room for the new fact that behaviour reaches this level among 

 the unicellulars. 



Among unicellulars, then, we see the beginning of explor- 

 ing and testing, the beginning of actual ' hunting ', and this 

 is on the main line of advance. We see, also, the enregis- 

 tration of particular reactions to stimuli, an organisation of 

 behaviour that is economical and in its rapidity of response 

 often life-saving. But we see, also, a selection of reactions, 

 a trying of one after the other till haply one meets the needs 

 of the case, and this ' trial-and-error ' method is likewise 

 on the main line of advance. 



4. Special Case of Shell-building among Arenaceous 

 Foraminifera. 



In the shell-building of some of the so-called arenaceous 

 Foraminifera Mr. E. A. Heron-Allen and Mr. A. Earland 

 have described (1915) what looks like constructive skill in 

 the use of materials. As every one knows, many unicellular 

 animals secrete shells of exquisite beauty, the ' organic crys- 

 tallisation ' of which is as much of an unsolved problem as 

 the adaptive internal architecture of bones, but in the case 

 of the arenaceous Foraminifera the building materials are 

 found ready-made in the environment and are utilised very 

 effectively to form a casing. The points of special interest 

 are two, (1) that a particular kind of material, such as 

 sponge-spicules, is selected from the surrounding debris, from 



