INSTINCTS OP UNICELLULAR ORGANISMS 91 



selection. For if we admit natural selection to be the cause of 

 the morphological adaptation in the interest of the species, it is 

 impossible to admit another origin for the correlative psycho- 

 logical adaptation, without which the former would be abso- 

 lutely useless. And not only is the instinct to be still a result 

 of selection, but also the instinct — of perhaps even greater im- 

 portance — of invariably choosing its habitat among the leaves 

 whose colouring corresponds to its own. 



There are also instincts among unicellular organisms in regard 

 to which consciousness may be entirely excluded as a factor in 

 their origin, for the organism is far too primitive for such a 

 psychical development to be thought of. Such is the instinct 

 of conjugation, as in the case of the Infusorians studied by Pro- 

 fessor Ischikawa. Of a similar nature are those extremely in- 

 teresting phenomena of nuclear conjugation which characterise 

 some of the simplest organisms — for instance, the Coccidium 

 proprium, a cell parasite of the salamander — which are nearly 

 as complicated as those to be found in the higher multicellular 

 organisms. The instinct to conjugate must be exclusively the 

 result of natural selection, since these micro-organisms are too 

 low down in the scale for consciousness to be thought of. 



We have already seen, in considering the Lamarckian hypo- 

 thesis of the inheritance of functional modifications, that several 

 categories of instincts cannot be explained as " inherited habits," 

 either because the concept of habit is irrelevant, as in the case 

 of instincts which manifest themselves only once during the life- 

 time of the individual ; or because hereditary transmission is im- 

 possible, as in the case of the sterile types with highly developed 

 instincts, such as the worker ants. But if we consider once 

 more the case of those instincts which are manifested but once 

 in a life-history, we find that consciousness may possibly have 

 had a share in their origin. The caterpillar which spins a silky 

 thread round its thorax in order to hang itself from a wall or 

 from a tree during the chrysalis stage must spin it so that it 



