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en actually converted into organs of flight。
In considering transitions of organs； it is so important to bear in mind the probability of conversion from one function to another； that I will give one more instance。 Pedunculated cirripedes have two minute folds of skin； called by me the ovigerous frena； which serve； through the means of a sticky secretion； to retain the eggs until they are hatched within the sack。 These cirripedes have no branchiae； the whole surface of the body and sack； including the small frena； serving for respiration。 The Balanidae or sessile cirripedes； on the other hand； have no ovigerous frena； the eggs lying loose at the bottom of the sack； in the well…enclosed shell； but they have large folded branchiae。 Now I think no one will dispute that the ovigerous frena in the one family are strictly homologous with the branchiae of the other family； indeed； they graduate into each other。 Therefore I do not doubt that little folds of skin； which originally served as ovigerous frena； but which； likewise； very slightly aided the act of respiration； have been gradually converted by natural selection into branchiae； simply through an increase in their size and the obliteration of their adhesive glands。 If all pedunculated cirripedes had become extinct； and they have already suffered far more extinction than have sessile cirripedes； who would ever have imagined that the branchiae in this latter family had originally existed as organs for preventing the ova from being washed out of the sack？
Although we must be extremely cautious in concluding that any organ could not possibly have been produced by successive transitional gradations； yet； undoubtedly； grave cases of difficulty occur； some of which will be discussed in my future work。
One of the gravest is that of neuter insects； which are often very differently constructed from either the males or fertile females； but this case will be treated of in the next chapter。 The electric organs of fishes offer another case of special difficulty； it is impossible to conceive by what steps these wondrous organs have been produced； but； as Owen and others have remarked； their intimate structure closely resembles that of common muscle； and as it has lately been shown that Rays have an organ closely analogous to the electric apparatus； and yet do not； as Matteuchi asserts； discharge any electricity； we must own that we are far too ignorant to argue that no transition of any kind is possible。
The electric organs offer another and even more serious difficulty； for they occur in only about a dozen fishes； of which several are widely remote in their affinities。 Generally when the same organ appears in several members of the same class； especially if in members having very different habits of life； we may attribute its presence to inheritance from a common ancestor； and its absence in some of the members to its loss through disuse or natural selection。 But if the electric organs had been inherited from one ancient progenitor thus provided； we might have expected that all electric fishes would have been specially related to each other。 Nor does geology at all lead to the belief that formerly most fishes had electric organs； which most of their modified descendants have lost。 The presence of luminous organs in a few insects； belonging to different families and orders； offers a parallel case of difficulty。 Other cases could be given； for instance in plants； the very curious contrivance of a mass of pollen…grains； borne on a foot…stalk with a sticky gland at the end； is the same in Orchis and Asclepias； genera almost as remote as possible amongst flowering plants。 In all these cases of two very distinct species furnished with apparently the same anomalous organ； it should be observed that； although the general appearance and function of the organ may be the same； yet some fundamental difference can generally be detected。 I am inclined to believe that in nearly the same way as two men have sometimes independently hit on the very same invention； so natural selection； working for the good of each being and taking advantage of analogous variations； has sometimes modified in very nearly the same manner two parts in two organic beings； which owe but little of their structure in common to inheritance from the same ancestor。
Although in many cases it is most difficult to conjecture by what transitions an organ could have arrived at its present state； yet； considering that the proportion of living and known forms to the extinct and unknown is very small； I have been astonished how rarely an organ can be named； towards which no transitional grade is known to lead。 The truth of this remark is indeed shown by that old canon in natural history of 'Natura non facit saltum。' We meet with this admission in the writings of almost every experienced naturalist； or； as Milne Edwards has well expressed it； nature is prodigal in variety； but niggard in innovation。 Why； on the theory of Creation； should this be so？ Why should all the parts and organs of many independent beings； each supposed to have been separately created for its proper place in nature； be so invariably linked together by graduated steps？ Why should not Nature have taken a leap from structure to structure？ On the theory of natural selection； we can clearly understand why she should not； for natural selection can act only by taking advantage of slight successive variations； she can never take a leap； but must advance by the shortest and slowest steps。
Organs of little apparent importance。 As natural selection acts by life and death； by the preservation of individuals with any favourable variation； and by the destruction of those with any unfavourable deviation of structure； I have sometimes felt much difficulty in understanding the origin of simple 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 complex 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 we know that the distribution and existence of cattle and other animals 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 great advantage。 It is not that the larger qu