496 LESLIE B. AREY AND W. J. CROZIER 
Onchidium, and Octopus exhibits more and more highly devel- 
oped 'homing' propensities. The return of a Patella, Fissurella, 
Siphonaria, or Calyptraea to its specific site cannot be accom- 
plished beyond a relatively slight distance; these creatures also 
tend to follow fairly definite paths in their excursions and to 
adhere to these paths when returning; and some of them creep 
but slightly, if at all, away from their scars. Onchidium's 
behavior is obviously an advance in respect to complexity. 
Analogous behavior has been described for snails and slugs (as 
in the famous story of the sick snail and its companion, cited 
by Darwin, '71, p. 316, and by others; cf. also Cooke, '95, and 
Scharff, '07). The investigation of this matter in snails and 
slugs holds the possibility of considerable interest. Finally, the 
behavior of Octopus (cf., e.g., Cowdry, '11), which returns to its 
nest after extensive forays and from considerable distances, 
under circumstances such that direct vision of the nest entrance 
is completely excluded, represents the most complex form of this 
activity among molluscs. 
There has been a tendency to regard any series of this kind as 
exhibiting stages in the evolution of a particular response, or 
even of an instinct. To speak of a 'homing instinct' is little 
short of a perversion of sense. Such a view-point is very hkely 
quite incorrect. Much more probable is it that this series of 
forms displays merely stages in the evolution of the central 
nervous machinery making possible more and more complicated 
behavior. The phrase 'evolution of an instinct' tends to obscure 
the real basis of the matter. Moreover, in the special instance 
under discussion, it is not at all obvious that the actual 'homing' 
performances of the several types named are in any sense geneti- 
cally connected; any relation with the mechanism of homing in 
higher forms, birds, for example, is in the highest degree improb- 
able. Even the homing of ants involves certain characteristics, 
such as those described by Cornetz ('14), which are not in any 
sense represented in the behavior of Onchidium. 
We early recognized the simulation of associative memory in 
the activities of Onchidium (Ai-ey and Crozier, '18) with refer- 
ence to its nest. If the notion of such memory or 'beginnings of 
