darwin and modern science-第14章
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e peculiar conditions of their life； and we must conclude that the earlier stages of these changes presented themselves to the processes of selection in the form of microscopic variations。 For it is as impossible to think of any origin other than through selection in this case as in the case of the toughness； and the 〃drip…tips〃 of tropical leaves。 And as these last could not have been produced directly by the beating of the heavy rain…drops upon them； so the calcareous anchors of Synapta cannot have been produced directly by the friction of the sand and mud at the bottom of the sea； and； since they are parts whose function is PASSIVE the Lamarckian factor of use and disuse does not come into question。 The conclusion is unavoidable； that the microscopically small variations of the calcareous bodies in the ancestral forms have been intensified and accumulated in a particular direction； till they have led to the formation of the anchor。 Whether this has taken place by the action of natural selection alone； or whether the laws of variation and the intimate processes within the germ…plasm have cooperated will become clear in the discussion of germinal selection。 This whole process of adaptation has obviously taken place within the time that has elapsed since this group of sea…cucumbers lost their tube…feet； those characteristic organs of locomotion which occur in no group except the Echinoderms； and yet have totally disappeared in the Synaptidae。 And after all what would animals that live in sand and mud do with tube…feet？
（c） COADAPTATION。
Darwin pointed out that one of the essential differences between artificial and natural selection lies in the fact that the former can modify only a few characters； usually only one at a time； while Nature preserves in the struggle for existence all the variations of a species； at the same time and in a purely mechanical way； if they possess selection…value。
Herbert Spencer； though himself an adherent of the theory of selection； declared in the beginning of the nineties that in his opinion the range of this principle was greatly over…estimated； if the great changes which have taken place in so many organisms in the course of ages are to be interpreted as due to this process of selection alone； since no transformation of any importance can be evolved by itself； it is always accompanied by a host of secondary changes。 He gives the familiar example of the Giant Stag of the Irish peat； the enormous antlers of which required not only a much stronger skull cap； but also greater strength of the sinews； muscles； nerves and bones of the whole anterior half of the animal； if their mass was not to weigh down the animal altogether。 It is inconceivable； he says； that so many processes of selection should take place SIMULTANEOUSLY； and we are therefore forced to fall back on the Lamarckian factor of the use and disuse of functional parts。 And how； he asks； could natural selection follow two opposite directions of evolution in different parts of the body at the same time； as for instance in the case of the kangaroo； in which the forelegs must have become shorter； while the hind legs and the tail were becoming longer and stronger？
Spencer's main object was to substantiate the validity of the Lamarckian principle； the cooperation of which with selection had been doubted by many。 And it does seem as though this principle； if it operates in nature at all； offers a ready and simple explanation of all such secondary variations。 Not only muscles； but nerves； bones； sinews； in short all tissues which function actively； increase in strength in proportion as they are used； and conversely they decrease when the claims on them diminish。 All the parts； therefore； which depend on the part that varied first； as for instance the enlarged antlers of the Irish Elk； must have been increased or decreased in strength； in exact proportion to the claims made upon them；just as is actually the case。
But beautiful as this explanation would be； I regard it as untenable； because it assumes the TRANSMISSIBILITY OF FUNCTIONAL MODIFICATIONS （so… called 〃acquired〃 characters）； and this is not only undemonstrable； but is scarcely theoretically conceivable； for the secondary variations which accompany or follow the first as correlative variations； occur also in cases in which the animals concerned are sterile and THEREFORE CANNOT TRANSMIT ANYTHING TO THEIR DESCENDANTS。 This is true of WORKER BEES； and particularly of ANTS； and I shall here give a brief survey of the present state of the problem as it appears to me。
Much has been written on both sides of this question since the published controversy on the subject in the nineties between Herbert Spencer and myself。 I should like to return to the matter in detail； if the space at my disposal permitted； because it seems to me that the arguments I advanced at that time are equally cogent to…day； notwithstanding all the objections that have since been urged against them。 Moreover； the matter is by no means one of subordinate interest； it is the very kernel of the whole question of the reality and value of the principle of selection。 For if selection alone does not suffice to explain 〃HARMONIOUS ADAPTATION〃 as I have called Spencer's COADAPTATION； and if we require to call in the aid of the Lamarckian factor it would be questionable whether selection could explain any adaptations whatever。 In this particular caseof worker bees the Lamarckian factor may be excluded altogether； for it can be demonstrated that here at any rate the effects of use and disuse cannot be transmitted。
But if it be asked why we are unwilling to admit the cooperation of the Darwinian factor of selection and the Lamarckian factor； since this would afford us an easy and satisfactory explanation of the phenomena； I answer： BECAUSE THE LAMARCKIAN PRINCIPLE IS FALLACIOUS； AND BECAUSE BY ACCEPTING IT WE CLOSE THE WAY TOWARDS DEEPER INSIGHT。 It is not a spirit of combativeness or a desire for self…vindication that induces me to take the field once more against the Lamarckian principle； it is the conviction that the progress of our knowledge is being obstructed by the acceptance of this fallacious principle； since the facile explanation it apparently affords prevents our seeking after a truer explanation and a deeper analysis。
The workers in the various species of ants are sterile； that is to say； they take no regular part in the reproduction of the species； although individuals among them may occasionally lay eggs。 In addition to this they have lost the wings； and the receptaculum seminis； and their compound eyes have degenerated to a few facets。 How could this last change have come about through disuse； since the eyes of workers are exposed to light in the same way as are those of the sexual insects and thus in this particular case are not liable to 〃disuse〃 at all？ The same is true of the receptaculum seminis； which can only have been disused as far as its glandular portion and its stalk are concerned； and also of the wings； the nerves tracheae and epidermal cells of which could not cease to function until the whole wing had degenerated； for the chitinous skeleton of the wing does not function at all in the active sense。
But； on the other hand； the workers in all species have undergone modifications in a positive direction； as； for instance； the greater development of brain。 In many species large workers have evolved；the so… called SOLDIERS； with enormous jaws and teeth； which defend the colony； and in others there are SMALL workers which have taken over other special functions； such as the rearing of the young Aphides。 This kind of division of the workers into two castes occurs among several tropical species of ants； but it is also present in the Italian species； Colobopsis truncata。 Beautifully as the size of the jaws could be explained as due to the increased use made of them by the 〃soldiers；〃 or the enlarged brain as due to the mental activities of the workers； the fact of the infertility of these forms is an insurmountable obstacle to accepting such an explanation。 Neither jaws nor brain can have been evolved on the Lamarckian principle。
The problem of coadaptation is no easier
