196 
DIFFICULTIES ON THEOKY. 
Chap. VI. 
former period, Lave 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 fiy-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 infiuence 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 
