is common to most animals。 On this view； the capacity of enduring the most different climates by man himself and by his domestic animals； and such facts as that former species of the elephant and rhinoceros were capable of enduring a glacial climate； whereas the living species are now all tropical or sub…tropical in their habits； ought not to be looked at as anomalies； but merely as examples of a very common flexibility of constitution； brought； under peculiar circumstances； into play。
How much of the acclimatisation of species to any peculiar climate is due to mere habit； and how much to the natural selection of varieties having different innate constitutions； and how much to means combined； is a very obscure question。 That habit or custom has some influence I must believe； both from analogy； and from the incessant advice given in agricultural works； even in the ancient Encyclopaedias of China； to be very cautious in transposing animals from one district to another； for it is not likely that man should have succeeded in selecting so many breeds and sub…breeds with constitutions specially fitted for their own districts： the result must； I think； be due to habit。 On the other hand； I can see no reason to doubt that natural selection will continually tend to preserve those individuals which are born with constitutions best adapted to their native countries。 In treatises on many kinds of cultivated plants； certain varieties are said to withstand certain climates better than others： this is very strikingly shown in works on fruit trees published in the United States； in which certain varieties are habitually recommended for the northern； and others for the southern States； and as most of these varieties are of recent origin； they cannot owe their constitutional differences to habit。 The case of the Jerusalem artichoke； which is never propagated by seed； and of which consequently new varieties have not been produced； has even been advanced for it is now as tender as ever it was as proving that acclimatisation cannot be effected！ The case； also； of the kidney…bean has been often cited for a similar purpose； and with much greater weight； but until some one will sow； during a score of generations； his kidney…beans so early that a very large proportion are destroyed by frost； and then collect seed from the few survivors； with care to prevent accidental crosses； and then again get seed from these seedlings； with the same precautions； the experiment cannot be said to have been even tried。 Nor let it be supposed that no differences in the constitution of seedling kidney…beans ever appear； for an account has been published how much more hardy some seedlings appeared to be than others。
On the whole； I think we may conclude that habit； use； and disuse； have； in some cases； played a considerable part in the modification of the constitution； and of the structure of various organs； but that the effects of use and disuse have often been largely combined with； and sometimes overmastered by； the natural selection of innate differences。
Correlation of Growth
I mean by this expression that the whole organisation is so tied together during its growth and development； that when slight variations in any one part occur； and are accumulated through natural selection； other parts become modified。 This is a very important subject； most imperfectly understood。 The most obvious case is； that modifications accumulated solely for the good of the young or larva； will； it may safely be concluded； affect the structure of the adult； in the same manner as any malconformation affecting the early embryo； seriously affects the whole organisation of the adult。 The several parts of the body which are homologous； and which； at an early embryonic period； are alike； seem liable to vary in an allied manner： we see this in the right and left sides of the body varying in the same manner； in the front and hind legs； and even in the jaws and limbs； varying together； for the lower jaw is believed to be homologous with the limbs。 These tendencies； I do not doubt； may be mastered more or less completely by natural selection： thus a family of stags once existed with an antler only on one side； and if this had been of any great use to the breed it might probably have been rendered permanent by natural selection。
Homologous parts； as has been remarked by some authors； tend to cohere； this is often seen in monstrous plants； and nothing is more common than the union of homologous parts in normal structures； as the union of the petals of the corolla into a tube。 Hard parts seem to affect the form of adjoining soft parts； it is believed by some authors that the diversity in the shape of the pelvis in birds causes the remarkable diversity in the shape of their kidneys。 Others believe that the shape of the pelvis in the human mother influences by pressure the shape of the head of the child。 In snakes； according to Schlegel； the shape of the body and the manner of swallowing determine the position of several of the most important viscera。
The nature of the bond of correlation is very frequently quite obscure。 M。 Is。 Geoffroy St Hilaire has forcibly remarked； that certain malconformations very frequently； and that others rarely coexist； without our being able to assign any reason。 What can be more singular than the relation between blue eyes and deafness in cats； and the tortoise…shell colour with the female sex； the feathered feet and skin between the outer toes in pigeons； and the presence of more or less down on the young birds when first hatched； with the future colour of their plumage； or； again； the relation between the hair and teeth in the naked Turkish dog； though here probably homology comes into play？ With respect to this latter case of correlation； I think it can hardly be accidental； that if we pick out the two orders of mammalia which are most abnormal in their dermal coverings； viz。 Cetacea （whales） and Edentata （armadilloes； scaly ant…eaters； &c。）； that these are likewise the most abnormal in their teeth。
I know of no case better adapted to show the importance of the laws of correlation in modifying important structures； independently of utility and； therefore； of natural selection； than that of the difference between the outer and inner flowers in some Compositous and Umbelliferous plants。 Every one knows the difference in the ray and central florets of； for instance； the daisy； and this difference is often accompanied with the abortion of parts of the flower。 But； in some Compositous plants； the seeds also differ in shape and sculpture； and even the ovary itself； with its accessory parts； differs； as has been described by Cassini。 These differences have been attributed by some authors to pressure； and the shape of the seeds in the ray…florets in some Compositae countenances this idea； but； in the case of the corolla of the Umbelliferae； it is by no means； as Dr Hooker informs me； in species with the densest heads that the inner and outer flowers most frequently differ。 It might have been thought that the development of the ray…petals by drawing nourishment from certain other parts of the flower had caused their abortion； but in some Compositae there is a difference in the seeds of the outer and inner florets without any difference in the corolla。 Possibly； these several differences may be connected with some difference in the flow of nutriment towards the central and external flowers： we know； at least； that in irregular flowers； those nearest to the axis are oftenest subject to peloria； and become regular。 I may add； as an instance of this； and of a striking case of correlation； that I have recently observed in some garden pelargoniums； that the central flower of the truss often loses the patches of darker colour in the two upper petals； and that when this occurs； the adherent nectary is quite aborted； when the colour is absent from only one of the two upper petals； the nectary is only much shortened。
With respect to the difference in the corolla of the central and exterior flowers of a head or umbel； I do not feel at all sure that C。 C。 Sprengel's idea that the ray…florets serve to attract insects； whose agency is highly advantageous in the fertilisation of plants of these two orders； is so far…fetched； as it may at first appear： and if it be advantageous； natural selection may have come into play。 But in regard to the differences both in the internal and external structure of the seeds； which are not always correlated with any differences in the flowers； it seems impossible that they can be in any way advantageous to the plant： yet in the Umbelliferae these differences are of such apparent importance the seeds being in some cases； according to Tausch； orthospermous in the exterior flowers and coelospermous in the central flowers； that the elder De Candolle founded his main divisions of the order on analogous differences。 Hence we see that modifications of structure； viewed by systematists as of high value； may be wholly due to unknown laws of correlated growth； and without being； as far as we can see； of the slightest service to the species。
We may often falsely attribute to correlation of growth； structures which are common to whole groups of species； and which in truth are simply due to inheritance； for an ancient progenitor may have acquired through natural selection some one modification in structure； and； after thousands of generations； some other and independent modification； and these two modifications； having been transmitted to a whole group of descendants with diverse habits； would naturally be thought to be correlated in some necessary manner。 So； again； I do not doubt that some apparent correlations； occurring throughout whole orders； are entirely due to the manner alone in which natural selection can act。 For instance； Alph。 De Candolle has remarked that winged seeds are never found in fruits which do not open： I should explain the rule by the fact that seeds could not gradually become winged through natural selection； except in fruits which opened； so that the individual plants producing seeds which were a little better fitted to be wafted further； might get an advantage over those producing seed less fitted for dispersal； and this process could not possi