ction； which favoured the longer rays in warm water； since they kept the organism afloat； while those with short rays sank and were eliminated。 If we put the question as to selection…value in this case； and ask how great the variations in the length of processes must be in order to possess selection…value； what can we answer except that these variations must have been minimal； and yet sufficient to prevent too rapid sinking and consequent elimination？ Yet this very case would give the ideal opportunity for a mathematical calculation of the minimal selection…value； although of course it is not feasible from lack of data to carry out the actual calculation。
But even in organisms of more than microscopic size there must frequently be minute； even microscopic differences which set going the process of selection； and regulate its progress to the highest possible perfection。
Many tropical trees possess thick； leathery leaves； as a protection against the force of the tropical rain drops。 The DIRECT influence of the rain cannot be the cause of this power of resistance； for the leaves； while they were still thin； would simply have been torn to pieces。 Their toughness must therefore be referred to selection； which would favour the trees with slightly thicker leaves； though we cannot calculate with any exactness how great the first stages of increase in thickness must have been。 Our hypothesis receives further support from the fact that； in many such trees； the leaves are drawn out into a beak…like prolongation （Stahl and Haberlandt） which facilitates the rapid falling off of the rain water； and also from the fact that the leaves； while they are still young； hang limply down in bunches which offer the least possible resistance to the rain。 Thus there are here three adaptations which can only be interpreted as due to selection。 The initial stages of these adaptations must undoubtedly have had selection…value。
But even in regard to this case we are reasoning in a circle； not giving 〃proofs；〃 and no one who does not wish to believe in the selection…value of the initial stages can be forced to do so。 Among the many pieces of presumptive evidence a particularly weighty one seems to me to be THE SMALLNESS OF THE STEPS OF PROGRESS which we can observe in certain cases； as for instance in leaf…imitation among butterflies； and in mimicry generally。 The resemblance to a leaf； for instance of a particular Kallima； seems to us so close as to be deceptive； and yet we find in another individual； or it may be in many others； a spot added which increases the resemblance； and which could not have become fixed unless the increased deceptiveness so produced had frequently led to the overlooking of its much persecuted possessor。 But if we take the selection…value of the initial stages for granted； we are confronted with the further question which I myself formulated many years ago： How does it happen THAT THE NECESSARY BEGINNINGS OF A USEFUL VARIATION ARE ALWAYS PRESENT？ How could insects which live upon or among green leaves become all green； while those that live on bark become brown？ How have the desert animals become yellow and the Arctic animals white？ Why were the necessary variations always present？ How could the green locust lay brown eggs； or the privet caterpillar develop white and lilac…coloured lines on its green skin？
It is of no use answering to this that the question is wrongly formulated （Plate； 〃Selektionsprinzip u。 Probleme der Artbildung〃 （3rd edition）； Leipzig； 1908。） and that it is the converse that is true； that the process of selection takes place in accordance with the variations that present themselves。 This proposition is undeniably true； but so also is another； which apparently negatives it： the variation required has in the majority of cases actually presented itself。 Selection cannot solve this contradiction； it does not call forth the useful variation； but simply works upon it。 The ultimate reason why one and the same insect should occur in green and in brown； as often happens in caterpillars and locusts； lies in the fact that variations towards brown presented themselves； and so also did variations towards green： THE KERNEL OF THE RIDDLE LIES IN THE VARYING； and for the present we can only say； that small variations in different directions present themselves in every species。 Otherwise so many different kinds of variations could not have arisen。 I have endeavoured to explain this remarkable fact by means of the intimate processes that must take place within the germ…plasm； and I shall return to the problem when dealing with 〃germinal selection。〃
We have； however； to make still greater demands on variation； for it is not enough that the necessary variation should occur in isolated individuals； because in that case there would be small prospect of its being preserved； notwithstanding its utility。 Darwin at first believed； that even single variations might lead to transformation of the species； but later he became convinced that this was impossible； at least without the cooperation of other factors； such as isolation and sexual selection。
In the case of the GREEN CATERPILLARS WITH BRIGHT LONGITUDINAL STRIPES； numerous individuals exhibiting this useful variation must have been produced to start with。 In all higher； that is； multicellular organisms； the germ…substance is the source of all transmissible variations； and this germ…plasm is not a simple substance but is made up of many primary constituents。 The question can therefore be more precisely stated thus： How does it come about that in so many cases the useful variations present themselves in numbers just where they are required； the white oblique lines in the leaf…caterpillar on the under surface of the body； the accompanying coloured stripes just above them？ And； further； how has it come about that in grass caterpillars； not oblique but longitudinal stripes； which are more effective for concealment among grass and plants； have been evolved？ And finally； how is it that the same Hawk…moth caterpillars； which to…day show oblique stripes； possessed longitudinal stripes in Tertiary times？ We can read this fact from the history of their development； and I have before attempted to show the biological significance of this change of colour。 （〃Studien zur Descendenz…Theorie〃 II。； 〃Die Enstehung der Zeichnung bei den Schmetterlings…raupen；〃 Leipzig； 1876。）
For the present I need only draw the conclusion that one and the same caterpillar may exhibit the initial stages of both； and that it depends on the manner in which these marking elements are INTENSIFIED and COMBINED by natural selection whether whitish longitudinal or oblique stripes should result。 In this case then the 〃useful variations〃 were actually 〃always there；〃 and we see that in the same group of Lepidoptera； e。g。 species of Sphingidae； evolution has occurred in both directions according to whether the form lived among grass or on broad leaves with oblique lateral veins； and we can observe even now that the species with oblique stripes have longitudinal stripes when young； that is to say； while the stripes have no biological significance。 The white places in the skin which gave rise； probably first as small spots； to this protective marking could be combined in one way or another according to the requirements of the species。 They must therefore either have possessed selection…value from the first； or； if this was not the case at their earliest occurrence； there must have been SOME OTHER FACTORS which raised them to the point of selection…value。 I shall return to this in discussing germinal selection。 But the case may be followed still farther； and leads us to the same alternative on a still more secure basis。
Many years ago I observed in caterpillars of Smerinthus populi （the poplar hawk…moth）； which also possess white oblique stripes； that certain individuals showed RED SPOTS above these stripes； these spots occurred only on certain segments； and never flowed together to form continuous stripes。 In another species （Smerinthus tiliae） similar blood…red spots unite to form a line…like coloured seam in the last stage of larval life； while in S。 ocellata rust…red spots appear in individual caterpillars； but more rarely than in S。 Populi； and they show no tendency to flow together。
Thus we have here the origin of a new character； arising from small beginnings； at least in S。 tiliae； in which species the coloured stripes are a normal specific character。 In the other species； S。 populi and S。 ocellata； we find the beginnings of the same variation； in one more rarely than in the other； and we can imagine that； in the course of time； in these two species； coloured lines over the oblique stripes will arise。 In any case these spots are the elements of variation； out of which coloured lines MAY be evolved； if they are combined in this direction through the agency of natural selection。 In S。 populi the spots are often small； but sometimes it seems as though several had united to form large spots。 Whether a process of selection in this direction will arise in S。 populi and S。 ocellata； 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 bod