224 ORGANIC EVOLUTION CONSIDERED 



periods of time. On the other hand, the great differ- 

 ences in the habits of spiders might be taken to show 

 that instincts change. 



Similarity of instincts, however, cannot be taken to 

 indicate, of necessity, a common origin. 



Mr. Darwin says that there exist " cases of instincts 

 almost identically the same in animals so remote in 

 the scale of Nature, that we cannot account for their 

 similarity by inheritance from a common progenitor, 

 and consequently must believe that they were inde- 

 pendently acquired through natural selection." * 



Again he says: "Many instincts are so wonderful 

 that their development will probably appear to the 

 reader a difficulty sufficient to overcome my whole 

 theory." 



Instances may be enumerated indefinitely Which 

 show the difficulty of accounting for changes of 

 structure and at the same time the origin of instincts 

 that render the acquired structures useful. Among 

 fishes, take the Sting Ray, which has a long prehensile 

 tail, and above the base of the tail a barbed spine 

 pointing backwards, on which it impales its enemies 

 by the use of its tail as a lasso. It seems to me en- 

 tirely improbably that the tail, the barb, and the 

 instinct to use them were evolved either simultaneous- 

 ly or in succession. 



How was the singular instinct evolved which causes 

 the male hornbills of Africa and India to plaster up 

 the female in a hole in a tree, leaving only a small 

 aperature through which they feed the female and 

 her youug? It is evident that this serves for protec- 

 tion, but I cannot see how it could have been grad- 

 ually evolved by natural selection. The work must 

 be quite complete before it can serve its purpose. 



In what way could the strange instinct of the Aus- 



* Origin oi Species, p. 226. 



