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Show 174 DIFFICULTIES ON THEORY. [CHAP. VI. independent beings, each supposed to have bee~ sep~rate\ Y created for its proper place in nature, be so 1nvanably hnked together by graduated steps? vVhy should not Nature have taken a leap from structure to structure ~ ·on the theory of natural selection, we can clearly understand why s_he should not ; for .natural sel~ction ?a~ act only by taking advantage of shght successive variations ; she can never take a leap, but must advance by the short-est and slowest steps. Organs of little apparent 11mportanae.-As natural selection acts by life and death,-by the preservation of individuals with any favorable variation, and by the destruction of those with any unfavorable deviation of structure,-! have sometimes felt much difficulty in understanding the origin of shnple parts, of which the importance does not seem sufficient to cause the preservation of successively varying individuals. I have sometimes felt as much difficulty, though of a very different kind, on this head, as in the case of an organ as perfect and com-plex as the eye. In the first place, we are much too ignorant in regard to the whole economy of any one organic being, to say what slight modifications would be of importance or not. · In a former chapter I have given instances of most trifling characters, such as the down on fruit and the colour of the flesh, which, from determining the attacks of insects or from being correlated with constitutional differences, might assuredly be acted on by natural selection. The tail of the giraffe looks like an artificially constructed fly-flapper; and it seems at first incredible that this could have been adapted for its present purpose by successive slight modifications, each better and better, for so trifling an object as-driving away flies; yet we should pause before being too positive even in this case, for 've know that the distribution and existence of cattle and other anhnals in South America absolutely depends on their power of resisting the attacks of insects: so that individuals which could by any means defend themselves from these small enemies, would be able to range into new pastures and thus gain a CHAP. VI.l ORGANS OF LITTLE IMPOR:TANCE. 175 great advantage. It is not that the larger quadrupeds are actually destroyed (except in some rare cases) by the flies but they are incessantly harassed and their strength re~ duced, so that they 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 in nearly the same state, although • now become of very slight use; and any actually injurious deviations in their structures 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 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 with the dog, though the aid must be slight, for the }}_are, with hardly any tail, can double quickly enough. In the second place, we may s~metimes 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 modification 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. |