IX.] RETROGRESSIVE DEVELOPMENT IN NATURE. 17 



ment of the eyes was above the average. This process of 

 natural selection would not only gradually produce improve- 

 ment in the eye, but would also tend to keep the improvement, 

 when gained, up to a certain standard. 



Now suppose such a species to have been carried under- 

 ground by water into a dark cavern. It would only graduallj'- 

 adapt itself to the new conditions and thus be enabled to thrive 

 in the cave : but after the lapse of generations the individuals 

 would have learnt to live in complete darkness, and to 

 distinguish and catch their prey without the aid of sight, and 

 this would be rendered possible by an improvement in other 

 organs, especially those of touch and smell. Thus in course of 

 time a race of newts would be produced perfectly adapted for 

 life in the dark, and for finding food by scent alone and not b}^ 

 sight ; and this race would make its way farther and farther 

 underground, and pass its whole life in utter darkness. It is in 

 some such way as this that not only the entrances of caverns, 

 but whole series of caves, connected by subterranean streams, 

 rivers and lakes, like those in the Karst Mountains, near Trieste, 

 have come to be tenanted by animals. 



Directly, however, such cave-dwellers became able to exist 

 without using their eyes, degeneration of these organs would 

 set in : as soon as they ceased to be essential to the life of the 

 animal, natural selection would be powerless to affect them, for 

 it would be immaterial whether the eyes of any animal were 

 above or below the standard. Hence the individuals with 

 weaker sight would no longer be eliminated, but would have an 

 equal chance of surviving and propagating their species. 

 Crossing would then take place between individuals with strong 

 and weak eyes, and the result would be a gradual deterioration 

 of the organ. Possibly the process might be accelerated by the 

 circumstance that small and degenerate eyes would be rather 

 an advantage, because their decrease would involve an increase 

 in the powers of other and now more important organs, such 

 as those of touch and smell. But even independently of this, 

 the eye, directly it ceases to be kept up to a certain standard of 

 development by natural selection, will gradually deteriorate, 

 the process being very slow at first, but absolutely sure. 



The same simple explanation suffices for all cases of retro- 

 gressive development, whether of organs or species. On any 



VOL. II. c 



