180 MENTAL EVOLUTION IN ANIMALS. 



" In another way we perhaps see the relation of habit and 

 instinct, namely in the latter acquiring great force if practised 

 only once or twice for a short time ; thus it is asserted that 

 if a calf or infant has never sucked its mother, it is very much 

 easier to bring it up by hand than if it has sucked only 

 once.* So again Kirbyf states that larva, after having ' fed 

 for a time on one plant, will die rather than eat another, 

 which would have been perfectly acceptable to them if ac- 

 customed to it from the first.' " 



Such, then, are some of the a priori reasons for believing 

 tliat instincts must have arisen from one or other of these 

 two sources — natural selection or lapsing intelligence ; it now 

 remains to prove, a posteriori, that they have so arisen. I 

 may first give a brief sketch of how this proof ought to 

 proceed. 



The proof, then, that instincts have had a primary mode 

 of origin requires to show : — 



I. That non-intelligent habits of a non-adaptive character 



occur in individuals. 



II. That such habits may be inherited. 



III. That such habits may vary. 



IV. That when they vary the variations may be inherited. 



V. That if such variations are inherited, we are justified 

 in assuming, in view of all that we know concerning 

 the analogous case of structures, that they may be fixed 

 and intensified in beneficial lines by natural selection. 



The proof that instincts have had a secondary mode of 

 origin requires to show : — 



VI. That intelligent adjustments when frequently per- 

 formed by the individual become automatic, either to 

 the extent of not requiring conscious thought at all, 

 or, as consciously adjustive habits, not requiring the 

 same degree of conscious effort as at first. 



VII. That automatic actions and conscious habits may 

 be inherited. 



Primary Instincts. 



Proceeding, then, to consider these sundry heads of proof, 

 it is easy to establish Proposition I, inasmuch as the fact 



Kirby and Spence, Entomology, voL i, p. 497. For the hammock caterpillar, 

 see Mem. Soc. Phys. de Geneve, tome vii, p. 154. 



* Zoonomia, p. 140. f Intro, to Entomol., vol. i, p. 391. 



