196 DIFFICULTIES ON THEORY. Chap. VI. 



former period, have been transmitted in nearly the same 

 state, although now become of very slight use ; and any 

 actually injurious deviations in their structure will always 

 have been checked by natural selection. Seeing how 

 important an organ of locomotion the tail is in most 

 aquatic animals, its general presence and use for many 

 purposes in so many land animals, which in their lungs 

 or modified swimbladders betray their aquatic origin, 

 may perhaps be thus accounted for. A well-developed 

 tail having been formed in an aquatic animal, it might 

 subsequently come to be worked in for all sorts of pur- 

 poses, as a fly-flapper, an organ of prehension, or as an 

 aid in turning, as with the dog, though the aid must be 

 slight, for the hare, with hardly any tail, can double 

 quickly enough. 



In the second place, we may sometimes attribute 

 importance to characters which are really of very little 

 importance, and which have originated from quite 

 secondary causes, independently of natural selection. 

 We should remember that climate, food, &c, probably 

 have some little direct influence on the organisation ; 

 that characters reappear from the law of reversion ; that 

 correlation of growth will have had a most important 

 influence in modifying various structures ; and finally, 

 that sexual selection will often have largely modified 

 the external characters of animals having a will, to 

 give one male an advantage in fighting with another 

 or in charming the females. Moreover when a modifi- 

 cation of structure has primarily arisen from the above 

 or other unknown causes, it may at first have been 

 of no advantage to the species, but may subsequently 

 have been taken advantage of by the descendants of 

 the species under new conditions of life and with newly 

 acquired habits. 



To give a few instances to illustrate these latter 



