384 MENTAL EVOLUTION IN ANIMALS. 



which moreover generally indicate, according to our theory, 

 the actual steps by which the instinct has been acquired, in 

 as much as we suppose allied instincts to have branched off 

 at different stages of descent from a common ancestor, and 

 therefore to have retained, more or less unaltered, the 

 instincts of the several lineal ancestral forms of any one 

 species : bearing all this in mind, together with the certainty 

 that instincts are as important to an animal as their generally 

 correlated structures, and that in the struggle for life under 

 changing conditions, slight modifications of instinct could 

 hardly fail occasionally to be profitable to individuals, I can 

 see no overwhelming difficulty on our theory. Even in the 

 most marvellous instinct known, that of the cells of the 

 Hive-bee, we have seen how a simple instinctive action may 

 lead to results which fill the mind with astonishment. 



Moreover it seems to me that the very general fact of the 

 gradation of complexity of instincts within the limits of the 

 same group of animals ; and likewise the fact of two allied 

 species, placed in tw^o distant parts of the world and sur- 

 rounded by wholly different conditions of life, still having 

 very much in common in their instincts, supports our theory 

 of descent ; for they are explained by it : whereas if we look 

 at each instinct as specially endowed, we can only say that 

 it is so. The imperfections and mistakes of instinct on our 

 theory cease to be surprising : indeed it would be wonderful 

 that far more numei'ous and flagrant cases could not be 

 detected, if it were not that a species which has failed to 

 become modified and so far perfected in its instincts that it 

 could continue struggling with the co-inhabitants of the same 

 region, would simply add one more to the myriads which 

 have become extinct. 



It may not be logical, but to my imagination, it is far more 

 satisfactory to look at the young cuckoo ejecting its foster- 

 brothers, ants making slaves, the larvae of the Ichneumidie 

 feeding within the live bodies of their prey, cats playing with 

 mice, otters and cormorants with living fish, not as instincts 

 specially given by the Creator, but as very small parts of one 

 general law leading to the advancement of all organic bodies 

 — Multiply, Vary, let the strongest Live and the weakest 

 Die. 



