52 THE DARWINIAN THEORY 



tendency to revert or slip back to a former and more 

 stable condition, and are kept with difficulty at the 

 stage they have reached. This is well seen in the 

 tendency which crossed pigeons have to revert to 

 the ancestral stage of the blue rock-pigeon. We 

 may illustrate this point by a simple mechanical 

 comparison. 



A pack of cards lying on a table are in a condition 

 of stability, and they may be taken to represent the 

 normal or ancestral condition. If now the cards are 

 built up to form a pagoda they are eminently, un- 

 stable, although forming a more imposing structure, 

 and are liable to collapse with the slightest touch, 

 and revert to their former condition of stability. So 

 the artificially-produced pigeons are much more im- 

 posing birds than the blue rock-pigeon, but they 

 are in a condition of great instability, and readily 

 revert to the ancestral condition. 

 ' Natural selection, on the other hand, acts, not for 

 the good of man, but for the good of the species, and 

 tends to preserve, develop, and perpetuate all 

 characters which will give the species an advantage 

 in the struggle for existence. There is no known 

 instance of an animal or plant having either structure 

 or instinct developed in order to benefit another 

 species. Every species is for itself and for itself 

 alone. 



