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； or whether it is now going on cannot be determined； since we cannot tell in advance what biological value the marking might have for these two species。 It is conceivable that the spots may have no selection… value as far as these species are concerned； and may therefore disappear again in the course of phylogeny； or； on the other hand； that they may be changed in another direction； for instance towards imitation of the rust… red fungoid patches on poplar and willow leaves。 In any case we may regard the smallest spots as the initial stages of variation； the larger as a cumulative summation of these。 Therefore either these initial stages must already possess selection…value； or； as I said before： THERE MUST BE SOME OTHER REASON FOR THEIR CUMULATIVE SUMMATION。 I should like to give one more example； in which we can infer； though we cannot directly observe； the initial stages。
All the Holothurians or sea…cucumbers have in the skin calcareous bodies of different forms； usually thick and irregular； which make the skin tough and resistant。 In a small group of themthe species of Synaptathe calcareous bodies occur in the form of delicate anchors of microscopic size。 Up till 1897 these anchors； like many other delicate microscopic structures； were regarded as curiosities； as natural marvels。 But a Swedish observer； Oestergren； has recently shown that they have a biological significance： they serve the footless Synapta as auxiliary organs of locomotion； since； when the body swells up in the act of creeping； they press firmly with their tips； which are embedded in the skin； against the substratum on which the animal creeps； and thus prevent slipping backwards。 In other Holothurians this slipping is made impossible by the fixing of the tube…feet。 The anchors act automatically； sinking their tips towards the ground when the corresponding part of the body thickens； and returning to the original position at an angle of 45 degrees to the upper surface when the part becomes thin again。 The arms of the anchor do not lie in the same plane as the shaft； and thus the curve of the arms forms the outermost part of the anchor； and offers no further resistance to the gliding of the animal。 Every detail of the anchor； the curved portion； the little teeth at the head； the arms； etc。； can be interpreted in the most beautiful way； above all the form of the anchor itself； for the two arms prevent it from swaying round to the side。 The position of the anchors； too； is definite and significant； they lie obliquely to the longitudinal axis of the animal； and therefore they act alike whether the animal is creeping backwards or forwards。 Moreover； the tips would pierce through the skin if the anchors lay in the longitudinal direction。 Synapta burrows in the sand； it first pushes in the thin anterior end； and thickens this again； thus enlarging the hole； then the anterior tentacles displace more sand； the body is worked in a little farther； and the process begins anew。 In the first act the anchors are passive； but they begin to take an active share in the forward movement when the body is contracted again。 Frequently the animal retains only the posterior end buried in the sand； and then the anchors keep it in position； and make rapid withdrawal possible。
Thus we have in these apparently random forms of the calcareous bodies； complex adaptations in which every little detail as to direction； curve； and pointing is exactly determined。 That they have selection…value in their present perfected form is beyond all doubt； since the animals are enabled by means of them to bore rapidly into the ground and so to escape from enemies。 We do not know what the initial stages were； but we cannot doubt that the little improvements； which occurred as variations of the originally simple slimy bodies of the Holothurians； were preserved because they already possessed selection…value for the Synaptidae。 For such minute microscopic structures whose form is so delicately adapted to the role they have to play in the life of the animal； cannot have arisen suddenly and as a whole； and every new variation of the anchor； that is； in the direction of the development of the two arms； and every curving of the shaft which prevented the tips from projecting at the wrong time； in short； every little adaptation in the modelling of the anchor must have possessed selection…value。 And that such minute changes of form fall within the sphere of fluctuating variations； that is to say； THAT THEY OCCUR is beyond all doubt。
In many of the Synaptidae the anchors are replaced by calcareous rods bent in the form of an S； which are said to act in the same way。 Others； such as those of the genus Ankyroderma； have anchors which project considerably beyond the skin； and； according to Oestergren； serve 〃to catch plant… particles and other substances〃 and so mask the animal。 Thus we see that in the Synaptidae the thick and irregular calcareous bodies of the Holothurians have been modified and transformed in various ways in adaptation to the footlessness of these animals； and to the 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 undemon