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Habit also has a deciding influence； as in the period of flowering with plants when transported from one climate to another。 In animals it has a more marked effect； for instance； I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more； in proportion to the whole skeleton； than do the same bones in the wild…duck； and I presume that this change may be safely attributed to the domestic duck flying much less； and walking more； than its wild parent。 The great and inherited development of the udders in cows and goats in countries where they are habitually milked； in comparison with the state of these organs in other countries； is another instance of the effect of use。 Not a single domestic animal can be named which has not in some country drooping ears； and the view suggested by some authors； that the drooping is due to the disuse of the muscles of the ear； from the animals not being much alarmed by danger； seems probable。
There are many laws regulating variation； some few of which can be dimly seen； and will be hereafter briefly mentioned。 I will here only allude to what may be called correlation of growth。 Any change in the embryo or larva will almost certainly entail changes in the mature animal。 In monstrosities； the correlations between quite distinct parts are very curious； and many instances are given in Isidore Geoffroy St Hilaire's great work on this subject。 Breeders believe that long limbs are almost always accompanied by an elongated head。 Some instances of correlation are quite whimsical； thus cats with blue eyes are invariably deaf； colour and constitutional peculiarities go together； of which many remarkable cases could be given amongst animals and plants。 From the facts collected by Heusinger； it appears that white sheep and pigs are differently affected from coloured individuals by certain vegetable poisons。 Hairless dogs have imperfect teeth； long…haired and coarse…haired animals are apt to have； as is asserted； long or many horns； pigeons with feathered feet have skin between their outer toes； pigeons with short beaks have small feet； and those with long beaks large feet。 Hence； if man goes on selecting； and thus augmenting； any peculiarity； he will almost certainly unconsciously modify other parts of the structure； owing to the mysterious laws of the correlation of growth。
The result of the various； quite unknown； or dimly seen laws of variation is infinitely complex and diversified。 It is well worth while carefully to study the several treatises published on some of our old cultivated plants； as on the hyacinth； potato； even the dahlia； &c。； and it is really surprising to note the endless points in structure and constitution in which the varieties and sub varieties differ slightly from each other。 The whole organization seems to have become plastic； and tends to depart in some small degree from that of the parental type。
Any variation which is not inherited is unimportant for us。 But the number and diversity of inheritable deviations of structure； both those of slight and those of considerable physiological importance； is endless。 Dr Prosper Lucas's treatise； in two large volumes； is the fullest and the best on this subject。 No breeder doubts how strong is the tendency to inheritance： like produces like is his fundamental belief： doubts have been thrown on this principle by theoretical writers alone。 When a deviation appears not unfrequently； and we see it in the father and child； we cannot tell whether it may not be due to the same original cause acting on both； but when amongst individuals； apparently exposed to the same conditions； any very rare deviation； due to some extraordinary combination of circumstances； appears in the parent say； once amongst several million individuals and it reappears in the child； the mere doctrine of chances almost compels us to attribute its reappearance to inheritance。 Every one must have heard of cases of albinism； prickly skin； hairy bodies； &c。 appearing in several members of the same family。 If strange and rare deviations of structure are truly inherited； less strange and commoner deviations may be freely admitted to be inheritable。 Perhaps the correct way of viewing the whole subject； would be； to look at the inheritance of every character whatever as the rule； and non…inheritance as the anomaly。
The laws governing inheritance are quite unknown； no one can say why the same peculiarity in different individuals of the same species； and in individuals of different species； is sometimes inherited and sometimes not so； why the child often reverts in certain characters to its grandfather or grandmother or other much more remote ancestor； why a peculiarity is often transmitted from one sex to both sexes or to one sex alone； more commonly but not exclusively to the like sex。 It is a fact of some little importance to us； that peculiarities appearing in the males of our domestic breeds are often transmitted either exclusively； or in a much greater degree； to males alone。 A much more important rule； which I think may be trusted； is that； at whatever period of life a peculiarity first appears； it tends to appear in the offspring at a corresponding age； though sometimes earlier。 In many cases this could not be otherwise： thus the inherited peculiarities in the horns of cattle could appear only in the offspring when nearly mature； peculiarities in the silkworm are known to appear at the corresponding caterpillar or cocoon stage。 But hereditary diseases and some other facts make me believe that the rule has a wider extension； and that when there is no apparent reason why a peculiarity should appear at any particular age； yet that it does tend to appear in the offspring at the same period at which it first appeared in the parent。 I believe this rule to be of the highest importance in explaining the laws of embryology。 These remarks are of course confined to the first appearance of the peculiarity； and not to its primary cause； which may have acted on the ovules or male element； in nearly the same manner as in the crossed offspring from a short…horned cow by a long…horned bull； the greater length of horn； though appearing late in life； is clearly due to the male element。
Having alluded to the subject of reversion； I may here refer to a statement often made by naturalists namely； that our domestic varieties； when run wild； gradually but certainly revert in character to their aboriginal stocks。 Hence it has been argued that no deductions can be drawn from domestic races to species in a state of nature。 I have in vain endeavoured to discover on what decisive facts the above statement has so often and so boldly been made。 There would be great difficulty in proving its truth： we may safely conclude that very many of the most strongly…marked domestic varieties could not possibly live in a wild state。 In many cases we do not know what the aboriginal stock was； and so could not tell whether or not nearly perfect reversion had ensued。 It would be quite necessary； in order to prevent the effects of intercrossing； that only a single variety should be turned loose in its new home。 Nevertheless； as our varieties certainly do occasionally revert in some of th