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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 possibly go on in fruit which did not open。
The elder Geoffroy and Goethe propounded； at about the same period； their law of compensation or balancement of growth； or； as Goethe expressed it； 'in order to spend on one side； nature is forced to economise on the other side。' I think this holds true to a certain extent with our domestic productions： if nourishment flows to one part or organ in excess； it rarely flows； at least in excess； to another part； thus it is difficult to get a cow to give much milk and to fatten readily。 The same varieties of the cabbage do not yield abundant and nutritious foliage and a copious supply of oil…bearing seeds。 When the seeds in our fruits become atrophied； the fruit itself gains largely in size and quality。 In our poultry； a large tuft of feathers on the head is generally accompanied by a diminished comb； and a large beard by diminished wattles。 With species in a state of nature it can hardly be maintained that the law is of universal application； but many good observers； more especially botanists； believe in its truth。 I will not； however； here give any instances； for I see hardly any way of distinguishing between the effects； on the one hand； of a part being largely developed through natural selection and another and adjoining part being reduced by this same process or by disuse； and； on the other hand； the actual withdrawal of nutriment from one part owing to the excess of growth in another and adjoining part。
I suspect； also； that some of the cases of compensation which have been advanced； and likewise some other facts； may be merged under a more general principle； namely； that natural selection is continually trying to economise in every part of the organisation。 If under changed conditions of life a structure before useful becomes less useful； any diminution； however slight； in its development； will be seized on by natural selection； for it will profit the individual not to have its nutriment wasted in building up an useless structure。