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the discussion light has been thrown on several facts； which on the theory of independent acts of creation are utterly obscure。 We have seen that species at any one period are not indefinitely variable； and are not linked together by a multitude of intermediate gradations； partly because the process of natural selection will always be very slow； and will act； at any one time； only on a very few forms； and partly because the very process of natural selection almost implies the continual supplanting and extinction of preceding and intermediate gradations。 Closely allied species； now living on a continuous area； must often have been formed when the area was not continuous； and when the conditions of life did not insensibly graduate away from one part to another。 When two varieties are formed in two districts of a continuous area； an intermediate variety will often be formed； fitted for an intermediate zone； but from reasons assigned； the intermediate variety will usually exist in lesser numbers than the two forms which it connects； consequently the two latter； during the course of further modification； from existing in greater numbers； will have a great advantage over the less numerous intermediate variety； and will thus generally succeed in supplanting and exterminating it。
We have seen in this chapter how cautious we should be in concluding that the most different habits of life could not graduate into each other； that a bat； for instance； could not have been formed by natural selection from an animal which at first could only glide through the air。
We have seen that a species may under new conditions of life change its habits； or have diversified habits； with some habits very unlike those of its nearest congeners。 Hence we can understand bearing in mind that each organic being is trying to live wherever it can live； how it has arisen that there are upland geese with webbed feet； ground woodpeckers； diving thrushes； and petrels with the habits of auks。
Although the belief that an organ so perfect as the eye could have been formed by natural selection； is more than enough to stagger any one； yet in the case of any organ； if we know of a long series of gradations in complexity； each good for its possessor； then； under changing conditions of life； there is no logical impossibility in the acquirement of any conceivable degree of perfection through natural selection。 In the cases in which we know of no intermediate or transitional states； we should be very cautious in concluding that none could have existed； for the homologies of many organs and their intermediate states show that wonderful metamorphoses in function are at least possible。 For instance； a swim…bladder has apparently been converted into an air…breathing lung。 The same organ having performed simultaneously very different functions； and then having been specialised for one function； and two very distinct organs having performed at the same time the same function； the one having been perfected whilst aided by the other； must often have largely facilitated transitions。
We are far too ignorant； in almost every case； to be enabled to assert that any part or organ is so unimportant for the welfare of a species； that modifications in its structure could not have been slowly accumulated by means of natural selection。 But we may confidently believe that many modifications； wholly due to the laws of growth； and at first in no way advantageous to a species； have been subsequently taken advantage of by the still further modified descendants of this species。 We may； also； believe that a part formerly of high importance has often been retained （as the tail of an aquatic animal by its terrestrial descendants）； though it has become of such small importance that it could not； in its present state； have been acquired by natural selection； a power which acts solely by the preservation of profitable variations in the struggle for life。
Natural selection will produce nothing in one species for the exclusive good or injury of another； though it may well produce parts； organs； and excretions highly useful or even indispensable； or highly injurious to another species； but in all cases at the same time useful to the owner。 Natural selection in each well…stocked country； must act chiefly through the competition of the inhabitants one with another； and consequently will produce perfection； or strength in the battle for life； only according to the standard of that country。 Hence the inhabitants of one country； generally the smaller one； will often yield； as we see they do yield； to the inhabitants of another and generally larger country。 For in the larger country there will have existed more individuals； and more diversified forms； and the competition will have been severer； and thus the standard of perfection will have been rendered higher。 Natural selection will not necessarily produce absolute perfection； nor； as far as we can judge by our limited faculties； can absolute perfection be everywhere found。
On the theory of natural selection we can clearly understand the full meaning of that old canon in natural history； 'Natura non facit saltum。' This canon； if we look only to the present inhabitants of the world； is not strictly correct； but if we include all those of past times； it must by my theory be strictly true。
It is generally acknowledged that all organic beings have been formed on two great laws Unity of Type； and the Conditions of Existence。 By unity of type is meant that fundamental agreement in structure； which we see in organic beings of the same class； and which is quite independent of their habits of life。 On my theory； unity of type is explained by unity of descent。 The expression of conditions of existence； so often insisted on by the illustrious Cuvier； is fully embraced by the principle of natural selection。 For natural selection acts by either now adapting the varying parts of each being to its organic and inorganic conditions of life； or by having adapted them during long…past periods of time： the adaptations being aided in some cases by use and disuse； being slightly affected by the direct action of the external conditions of life； and being in all cases subjected to the several laws of growth。 Hence； in fact； the law of the Conditions of Existence is the higher law； as it includes； through the inheritance of former adaptations； that of Unity of Type。
Chapter 7 … Instinct
THE subject of instinct might have been worked into the previous chapters； but I have thought that it would be more convenient to treat the subject separately； especially as so wonderful an instinct as that of the hive…bee making its cells will probably have occurred to many readers； as a difficulty sufficient to overthrow my whole theory。 I must premise； that I have nothing to do with the origin of the primary mental powers； any more than I have with that of life itself。 We are concerned only with the diversities of instinct and of the other mental qualities of animals within the same class。
I will not attempt any definition of instinct。 It would be easy to show that several distinct mental actions are commonly embraced by this term； but every one understands what is meant； when it is sai