176 ORGANS OF LITTLE IMPORTANCE 



are more subject to disease, or not so well enabled in a 

 coming dearth to search for food, or to escape from beasts 

 of prey. 



Organs now of trifling importance have probably in some 

 cases been of high importance to an early progenitor, and, 

 after having been slowly perfected at a former period, have 

 been transmitted to existing species in nearly the same 

 state, although now of very slight use ; but any actually 

 injurious deviations in their structure would of course 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 swim-bladders 

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

 as an aid in turning, as in the case of the dog, though the 

 aid in this latter respect must be slight, for the hare, with 

 hardly any tail, can double still more quickly. 



In the second place, we may easily err in attributing 

 importance to characters, and in believing that they have 

 been developed through natural selection. We must by no 

 means overlook the effects of the definite action of changed 

 conditions of life, of so-called spontaneous variations, which 

 seem to depend in a quite subordinate degree on the nature 

 of the conditions, of the tendency to reversion to long^ost 

 characters, of the complex laws of growth, such as of correla- 

 tion, comprehension, of the pressure of one part on another, 

 etc., and finally of sexual selection, by which characters of 

 use to one sex are often gained and then transmitted more 

 or less perfectly to the other sex, though of no use to the 

 sex. But structures thus indirectly gained, although at 

 first of no advantage to a species, may subsequently have 

 been taken advantage of by its modified descendants, under- 

 new conditions of life and newly acquired habits. 



If green woodpeckers alone had existed, and we did not 

 know that there were many black and pied kinds, I dare 

 say that we should have thought that the green color was a 

 beautiful adaptation to conceal this tree-frequenting bird 

 from its enemies; and consequently that it was a character 

 of importance and had been acquired through natural selec- 

 tion ; as it is, the color is probably in ch>ef part due to 

 sexual selection, A trailing palm in the Malay Archipelago 



